© 2015 Pearson Education, Inc.

Elements, Atoms, and Compounds © 2015 Pearson Education, Inc.

2.1 Organisms are composed of elements, in combinations called compounds Living organisms are composed of matter, which is anything that occupies space and has mass (weight). Matter is composed of chemical elements. An element is a substance that cannot be broken down to other substances by ordinary chemical means. There are 92 elements in nature—only a few exist in a pure state. Student Misconceptions and Concerns • The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips • The text notes the unique properties of pure sodium, pure chlorine, and the compound sodium chloride formed when the two bond together. Consider challenging your students to think of other simple examples of new properties that result when a compound is formed. (For example, water, formed from hydrogen and oxygen, and rust, formed from iron and oxygen.) • Students might be interested in the following aside: One of the challenges of raising captive, exotic animals is meeting the unique dietary requirements of a species. A zoo may struggle to keep a particular animal because zoologists have not identified all of the trace elements required in the animal’s diet. © 2015 Pearson Education, Inc.

Table 2.1 Table 2.1 Elements in the human body

2.1 Organisms are composed of elements, in combinations called compounds A compound is a substance consisting of two or more different elements in a fixed ratio. Compounds are more common than pure elements. Sodium chloride, table salt, is a common compound of equal parts of sodium (Na) and chlorine (Cl). Student Misconceptions and Concerns • The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips • The text notes the unique properties of pure sodium, pure chlorine, and the compound sodium chloride formed when the two bond together. Consider challenging your students to think of other simple examples of new properties that result when a compound is formed. (For example, water, formed from hydrogen and oxygen, and rust, formed from iron and oxygen.) • Students might be interested in the following aside: One of the challenges of raising captive, exotic animals is meeting the unique dietary requirements of a species. A zoo may struggle to keep a particular animal because zoologists have not identified all of the trace elements required in the animal’s diet. © 2015 Pearson Education, Inc.

Sodium (Na) Chlorine (Cl) Sodium chloride (NaCl) Figure 2.1-0 Figure 2.1-0 The emergent properties of the edible compound sodium chloride Sodium (Na) Chlorine (Cl) Sodium chloride (NaCl)

2.1 Organisms are composed of elements, in combinations called compounds About 25 elements are essential for human life. Four elements make up about 96% of the weight of most living organisms. These are oxygen, carbon, hydrogen, and nitrogen. Trace elements are essential but are only needed in minute quantities. Student Misconceptions and Concerns • The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips • The text notes the unique properties of pure sodium, pure chlorine, and the compound sodium chloride formed when the two bond together. Consider challenging your students to think of other simple examples of new properties that result when a compound is formed. (For example, water, formed from hydrogen and oxygen, and rust, formed from iron and oxygen.) • Students might be interested in the following aside: One of the challenges of raising captive, exotic animals is meeting the unique dietary requirements of a species. A zoo may struggle to keep a particular animal because zoologists have not identified all of the trace elements required in the animal’s diet. © 2015 Pearson Education, Inc.

2.2 CONNECTION: Trace elements are common additives to food and water Some trace elements are required to prevent disease. Without iron, your body cannot transport oxygen. An iodine deficiency prevents production of thyroid hormones, resulting in goiter. Student Misconceptions and Concerns • The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips • Students might be interested in the following aside: One of the challenges of raising captive, exotic animals is meeting the unique dietary requirements of a species. A zoo may struggle to keep a particular animal because zoologists have not identified all of the trace elements required in the animal’s diet. • Many breakfast cereals are fortified with iron (see Figure 2.2c). As noted in Module 2.2, you can crush the cereal and extract distinct iron particles with a magnet. An overhead projector or video imaging device should clearly reveal the iron particles stuck to the magnet. This short practical demonstration can help connect a concrete example to an abstract concept. © 2015 Pearson Education, Inc.

Figure 2.2a Figure 2.2a Goiter, a symptom of iodine deficiency, in a Burmese woman

2.2 CONNECTION: Trace elements are common additives to food and water Fluoride is usually added to municipal water and dental products to help reduce tooth decay. Student Misconceptions and Concerns • The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips • Students might be interested in the following aside: One of the challenges of raising captive, exotic animals is meeting the unique dietary requirements of a species. A zoo may struggle to keep a particular animal because zoologists have not identified all of the trace elements required in the animal’s diet. • Many breakfast cereals are fortified with iron (see Figure 2.2c). As noted in Module 2.2, you can crush the cereal and extract distinct iron particles with a magnet. An overhead projector or video imaging device should clearly reveal the iron particles stuck to the magnet. This short practical demonstration can help connect a concrete example to an abstract concept. © 2015 Pearson Education, Inc.

2.2 CONNECTION: Trace elements are common additives to food and water Several chemicals are added to food to help preserve it, make it more nutritious, and/or make it look better. Check out the nutrition facts label on foods and drinks you purchase. Student Misconceptions and Concerns • The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips • Students might be interested in the following aside: One of the challenges of raising captive, exotic animals is meeting the unique dietary requirements of a species. A zoo may struggle to keep a particular animal because zoologists have not identified all of the trace elements required in the animal’s diet. • Many breakfast cereals are fortified with iron (see Figure 2.2c). As noted in Module 2.2, you can crush the cereal and extract distinct iron particles with a magnet. An overhead projector or video imaging device should clearly reveal the iron particles stuck to the magnet. This short practical demonstration can help connect a concrete example to an abstract concept. © 2015 Pearson Education, Inc.

Figure 2.2c Figure 2.2c Nutrition facts from a fortified cereal

2.3 Atoms consist of protons, neutrons, and electrons Each element consists of one kind of atom. An atom is the smallest unit of matter that still retains the properties of an element. Three subatomic particles in atoms are relevant to our discussion of the properties of elements. Protons are positively charged. Electrons are negatively charged. Neutrons are electrically neutral. Student Misconceptions and Concerns • The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. • Students with limited backgrounds in chemistry and physics might struggle with basic concepts of mass, weight, compounds, elements, and isotopes. It may also be early in the semester when mature study habits have not yet developed. Consider passing along basic studying advice and tips to help students master these early chemistry concepts. In-class quizzes (graded or not) or a few homework problems will also provide reinforcing practice. Teaching Tips • Here is a comparison that helps make the point about the differences in mass of protons and electrons. If a proton were as massive as a bowling ball, an electron would be the mass of a Life Saver candy. (This is calculated by considering a 15-pound bowling ball, a Life Saver candy with a mass of 0.12 ounces, and the mention in Module 2.3 that an electron is about 1/2,000 the mass of a proton.) • The text in Module 2.3 makes an analogy regarding the size of a helium atom. The text notes that if a helium atom were the size of Yankee Stadium, the nucleus would be about the size of a pea in center field, and the two electrons would be like tiny gnats buzzing around the stadium. This analogy helps to relate the great distances between parts of an atom. Consider modifying the analogy to any local stadium in your region. Such concrete examples help to relate abstract concepts. • After sharing the tips above, consider asking your students to compare the mass of the gnat orbiting Yankee Stadium to the mass of a pea in center field, assuming the proportional differences between the mass of an electron and mass of a proton. If a proton or neutron is about 2,000 times more massive than an electron, how does the mass of a helium nucleus compare to the mass of one of its electrons? • The text notes the use of radioactive isotopes in dating fossils but references Module 15.5 for further discussion. If your course does not include Chapter 15, consider discussing the use of half-lives to date fossils. Note: Many radioactive isotopes, with great variation in half-lives, are used to date fossils. Active Lecture Tips • See the essay Electrons and Ceiling Fans on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

2.3 Atoms consist of protons, neutrons, and electrons Neutrons and protons are packed into an atom’s nucleus. Electrons orbit the nucleus. The negative charge of electrons and the positive charge of protons keep electrons near the nucleus. Student Misconceptions and Concerns • The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. • Students with limited backgrounds in chemistry and physics might struggle with basic concepts of mass, weight, compounds, elements, and isotopes. It may also be early in the semester when mature study habits have not yet developed. Consider passing along basic studying advice and tips to help students master these early chemistry concepts. In-class quizzes (graded or not) or a few homework problems will also provide reinforcing practice. Teaching Tips • Here is a comparison that helps make the point about the differences in mass of protons and electrons. If a proton were as massive as a bowling ball, an electron would be the mass of a Life Saver candy. (This is calculated by considering a 15-pound bowling ball, a Life Saver with a mass of 0.12 ounces, and the mention in Module 2.3 that an electron is about 1/2,000 the mass of a proton.) • The text in Module 2.3 makes an analogy regarding the size of a helium atom. The text notes that if a helium atom were the size of Yankee Stadium, the nucleus would be about the size of a pea in center field, and the two electrons would be like tiny gnats buzzing around the stadium. This analogy helps to relate the great distances between parts of an atom. Consider modifying the analogy to any local stadium in your region. Such concrete examples help to relate abstract concepts. • After sharing the tips above, consider asking your students to compare the mass of the gnat orbiting Yankee Stadium to the mass of a pea in center field, assuming the proportional differences between the mass of an electron and mass of a proton. If a proton or neutron is about 2,000 times more massive than an electron, how does the mass of a helium nucleus compare to the mass of one of its electrons? • The text notes the use of radioactive isotopes in dating fossils but references Module 15.5 for further discussion. If your course does not include Chapter 15, consider discussing the use of half-lives to date fossils. Note: Many radioactive isotopes, with great variation in half-lives, are used to date fossils. Active Lecture Tips • See the essay Electrons and Ceiling Fans on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

− − + + + + Nucleus 2e− Electron cloud 2 + Protons Nucleus 2 Neutrons Figure 2.3 − − Nucleus 2e− + + + + Electron cloud Figure 2.3 Two models of a helium atom 2 + Protons Nucleus 2 Neutrons 2 − Electrons

2.3 Atoms consist of protons, neutrons, and electrons The number of protons is the atom’s atomic number. An atom’s mass number is the sum of the number of protons and neutrons in the nucleus. The atomic mass is approximately equal to its mass number. Student Misconceptions and Concerns • The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. • Students with limited backgrounds in chemistry and physics might struggle with basic concepts of mass, weight, compounds, elements, and isotopes. It may also be early in the semester when mature study habits have not yet developed. Consider passing along basic studying advice and tips to help students master these early chemistry concepts. In-class quizzes (graded or not) or a few homework problems will also provide reinforcing practice. Teaching Tips • Here is a comparison that helps make the point about the differences in mass of protons and electrons. If a proton were as massive as a bowling ball, an electron would be the mass of a Life Saver candy. (This is calculated by considering a 15-pound bowling ball, a Life Saver with a mass of 0.12 ounces, and the mention in Module 2.3 that an electron is about 1/2,000 the mass of a proton.) • The text in Module 2.3 makes an analogy regarding the size of a helium atom. The text notes that if a helium atom were the size of Yankee Stadium, the nucleus would be about the size of a pea in center field, and the two electrons would be like tiny gnats buzzing around the stadium. This analogy helps to relate the great distances between parts of an atom. Consider modifying the analogy to any local stadium in your region. Such concrete examples help to relate abstract concepts. • After sharing the tips above, consider asking your students to compare the mass of the gnat orbiting Yankee Stadium to the mass of a pea in center field, assuming the proportional differences between the mass of an electron and mass of a proton. If a proton or neutron is about 2,000 times more massive than an electron, how does the mass of a helium nucleus compare to the mass of one of its electrons? • The text notes the use of radioactive isotopes in dating fossils but references Module 15.5 for further discussion. If your course does not include Chapter 15, consider discussing the use of half-lives to date fossils. Note: Many radioactive isotopes, with great variation in half-lives, are used to date fossils. Active Lecture Tips • See the essay Electrons and Ceiling Fans on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

− − + + + + Nucleus 2e− Electron cloud 2 + Protons Nucleus 2 Neutrons Figure 2.3 − − Nucleus 2e− + + + + Electron cloud Figure 2.3 Two models of a helium atom 2 + Protons Nucleus 2 Neutrons 2 − Electrons

2.3 Atoms consist of protons, neutrons, and electrons Although all atoms of an element have the same atomic number, some differ in mass number. Different isotopes of an element have the same number of protons but different numbers of neutrons. Different isotopes of an element behave identically in chemical reactions. In radioactive isotopes, the nucleus decays spontaneously, giving off particles and energy. Student Misconceptions and Concerns • The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. • Students with limited backgrounds in chemistry and physics might struggle with basic concepts of mass, weight, compounds, elements, and isotopes. It may also be early in the semester when mature study habits have not yet developed. Consider passing along basic studying advice and tips to help students master these early chemistry concepts. In-class quizzes (graded or not) or a few homework problems will also provide reinforcing practice. Teaching Tips • Here is a comparison that helps make the point about the differences in mass of protons and electrons. If a proton were as massive as a bowling ball, an electron would be the mass of a Life Saver candy. (This is calculated by considering a 15-pound bowling ball, a Life Saver with a mass of 0.12 ounces, and the mention in Module 2.3 that an electron is about 1/2,000 the mass of a proton.) • The text in Module 2.3 makes an analogy regarding the size of a helium atom. The text notes that if a helium atom were the size of Yankee Stadium, the nucleus would be about the size of a pea in center field, and the two electrons would be like tiny gnats buzzing around the stadium. This analogy helps to relate the great distances between parts of an atom. Consider modifying the analogy to any local stadium in your region. Such concrete examples help to relate abstract concepts. • After sharing the tips above, consider asking your students to compare the mass of the gnat orbiting Yankee Stadium to the mass of a pea in center field, assuming the proportional differences between the mass of an electron and mass of a proton. If a proton or neutron is about 2,000 times more massive than an electron, how does the mass of a helium nucleus compare to the mass of one of its electrons? • The text notes the use of radioactive isotopes in dating fossils but references Module 15.5 for further discussion. If your course does not include Chapter 15, consider discussing the use of half-lives to date fossils. Note: Many radioactive isotopes, with great variation in half-lives, are used to date fossils. Active Lecture Tips • See the essay Electrons and Ceiling Fans on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

Table 2.3 Table 2.3 Isotopes of carbon

2.4 CONNECTION: Radioactive isotopes can help or harm us Living cells cannot distinguish between isotopes of the same element. Therefore, radioactive compounds in metabolic processes can act as tracers. This radioactivity can be detected by instruments. By using these instruments, the fate of radioactive tracers can be monitored in living organisms. Student Misconceptions and Concerns • The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips • The half-lives of many radioactive substances, especially those used for dating fossils, might lead some students to expect very long periods of decay for any radioactive substance. This might even be alarming if students are someday asked to consume a radioactive substance for a medical test. However, some medically significant isotopes have relatively short half-lives. Radioactive iodine-131 is often used to diagnose or treat certain thyroid problems. Its half-life of eight days means that it will decay quickly. • Depending upon where you are teaching, radon in homes may be a common problem and pose a significant health risk. If you are in a high-radon region, consider adding details about home remediation methods and expenses or have students research the topic and report back. © 2015 Pearson Education, Inc.

2.4 CONNECTION: Radioactive isotopes can help or harm us Radioactive tracers are frequently used in medical diagnosis. Sophisticated imaging instruments are used to detect them. An imaging instrument that uses positron-emission tomography (PET) detects the location of injected radioactive materials. PET is useful for diagnosing heart disorders and cancer and in brain research. Student Misconceptions and Concerns • The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips • The half-lives of many radioactive substances, especially those used for dating fossils, might lead some students to expect very long periods of decay for any radioactive substance. This might even be alarming if students are someday asked to consume a radioactive substance for a medical test. However, some medically significant isotopes have relatively short half-lives. Radioactive iodine-131 is often used to diagnose or treat certain thyroid problems. Its half-life of eight days means that it will decay quickly. • Depending upon where you are teaching, radon in homes may be a common problem and pose a significant health risk. If you are in a high-radon region, consider adding details about home remediation methods and expenses or have students research the topic and report back. © 2015 Pearson Education, Inc.

Healthy person Alzheimer’s patient Figure 2.4b Figure 2.4b PET images of brains of a healthy person (left) and a person with Alzheimer’s disease (right) Healthy person Alzheimer’s patient

2.4 CONNECTION: Radioactive isotopes can help or harm us In addition to benefits, there are also dangers associated with using radioactive substances. Uncontrolled exposure can cause damage to some molecules in a living cell, especially DNA. Chemical bonds are broken by the emitted energy, which causes abnormal bonds to form. Student Misconceptions and Concerns • The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips • The half-lives of many radioactive substances, especially those used for dating fossils, might lead some students to expect very long periods of decay for any radioactive substance. This might even be alarming if students are someday asked to consume a radioactive substance for a medical test. However, some medically significant isotopes have relatively short half-lives. Radioactive iodine-131 is often used to diagnose or treat certain thyroid problems. Its half-life of eight days means that it will decay quickly. • Depending upon where you are teaching, radon in homes may be a common problem and pose a significant health risk. If you are in a high-radon region, consider adding details about home remediation methods and expenses or have students research the topic and report back. © 2015 Pearson Education, Inc.

Chemical Bonds © 2015 Pearson Education, Inc.

2.5 The distribution of electrons determines an atom’s chemical properties Of the three subatomic particles—protons, neutrons, and electrons—only electrons are directly involved in the chemical activity of an atom. Electrons can be located in different electron shells, each with a characteristic distance from the nucleus. An atom may have one, two, or more electron shells. Information about the distribution of electrons is found in the periodic table of the elements. Active Lecture Tips  Ask students to use 3 to 4 minutes to turn to someone near them to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Nearby pairs might provide immediate critiques with each other before passing along analogies for the entire class to consider. After these discussions, have pairs of students share their analogies with the entire class. Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and critical thinking skills. For example, ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both men look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) • See the essay Relating Chemical Bonds to Everyday Ideas on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

Figure 2.5a Figure 2.5a An electron distribution model of carbon

First shell Second shell Third shell Figure 2.5b-0 Hydrogen Helium First shell Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Second shell Sodium Magnesium Aluminum Silicon Phosphorus Sulfur Chlorine Argon Third shell Figure 2.5b-0 The electron distribution diagrams of the first 18 elements in the periodic table

2.5 The distribution of electrons determines an atom’s chemical properties The number of electrons in the outermost shell, called the valence shell, determines the chemical properties of the atom. Atoms whose outer shells are not full tend to interact with other atoms in ways that enable them to complete or fill their valence shells. Active Lecture Tips  Ask students to use 3 to 4 minutes to turn to someone near them to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Nearby pairs might provide immediate critiques with each other before passing along analogies for the entire class to consider. After these discussions, have pairs of students share their analogies with the entire class. Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and critical thinking skills. For example, ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both men look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) • See the essay Relating Chemical Bonds to Everyday Ideas on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

2.5 The distribution of electrons determines an atom’s chemical properties When two atoms with incomplete outer shells react, each atom will share, donate, or receive electrons, so that both partners end up with completed outer shells. These interactions usually result in atoms staying close together, held by attractions called chemical bonds. Active Lecture Tips  Ask students to use 3 to 4 minutes to turn to someone near them to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Nearby pairs might provide immediate critiques with each other before passing along analogies for the entire class to consider. After these discussions, have pairs of students share their analogies with the entire class. Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and critical thinking skills. For example, ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both men look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) • See the essay Relating Chemical Bonds to Everyday Ideas on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

2.6 Covalent bonds join atoms into molecules through electron sharing In a covalent bond, two atoms, each with an unpaired electron in its outer shell, actually share a pair of electrons. Two or more atoms held together by covalent bonds form a molecule. A covalent bond connects two hydrogen atoms in a molecule of the gas H2. Student Misconceptions and Concerns • Students with limited backgrounds in chemistry will benefit from a discussion of Figure 2.6 and the differences and limitations of representing atomic structure. The contrast in Figure 2.6 is a good beginning for such a discussion. In addition to comparing how the positions of electrons are depicted, note the problems with the sense of scale as discussed in Module 2.3. Teaching Tips • Have your students try to calculate the number of covalent bonds possible for a variety of atoms. (Carbon, for example, can form up to four covalent bonds.) Then provide the students with a list of elements and the number of outer electrons for each and have them make predictions about the chemical formula for many types of molecules. (For example, carbon could form covalent bonds with four hydrogen atoms.) • Modules 2.6 and 2.8 discuss the special bonding in and between water molecules. Many students do not appreciate the importance of weak chemical bonds in water and cellular chemistry. Extra time and attention may be required to address this special aspect of chemistry. Active Lecture Tips  Ask students to use 3 to 4 minutes to turn to someone near them to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Nearby pairs might provide immediate critiques with each other before passing along analogies for the entire class to consider. After these discussions, have pairs of students share their analogies with the entire class. Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and critical thinking skills. For example, ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both men look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) • See the essay Relating Chemical Bonds to Everyday Ideas on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

2.6 Covalent bonds join atoms into molecules through electron sharing There are four alternative ways to represent common molecules. The hydrogen atoms in H2 are held together by a pair of shared electrons. In an oxygen molecule (O2), the two oxygen atoms share two pairs of electrons, forming a double bond, indicated in a structural formula by a pair of lines. Student Misconceptions and Concerns • Students with limited backgrounds in chemistry will benefit from a discussion of Figure 2.6 and the differences and limitations of representing atomic structure. The contrast in Figure 2.6 is a good beginning for such a discussion. In addition to comparing how the positions of electrons are depicted, note the problems with the sense of scale as discussed in Module 2.3. Teaching Tips • Have your students try to calculate the number of covalent bonds possible for a variety of atoms. (Carbon, for example, can form up to four covalent bonds.) Then provide the students with a list of elements and the number of outer electrons for each and have them make predictions about the chemical formula for many types of molecules. (For example, carbon could form covalent bonds with four hydrogen atoms.) • Modules 2.6 and 2.8 discuss the special bonding in and between water molecules. Many students do not appreciate the importance of weak chemical bonds in water and cellular chemistry. Extra time and attention may be required to address this special aspect of chemistry. Active Lecture Tips  Ask students to use 3 to 4 minutes to turn to someone near them to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Nearby pairs might provide immediate critiques with each other before passing along analogies for the entire class to consider. After these discussions, have pairs of students share their analogies with the entire class. Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and critical thinking skills. For example, ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both men look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) • See the essay Relating Chemical Bonds to Everyday Ideas on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

H2 Hydrogen Single bond O2 Oxygen Double bond Figure 2.6-1 Molecular Formula Electron Distribution Diagram Structural Formula Space-Filling Model H2 Hydrogen Single bond H H O2 Oxygen Double bond Figure 2.6-1 Alternative ways to represent four common molecules (part 1) O O

CH4 Methane Nonpolar covalent bonds H2O Water Polar covalent bonds Figure 2.6-2 Molecular Formula Electron Distribution Diagram Structural Formula Space-Filling Model H CH4 Methane H C H Nonpolar covalent bonds H Figure 2.6-2 Alternative ways to represent four common molecules (part 2) H2O Water O Polar covalent bonds H H

(slightly −) (slightly +) (slightly +) Polar covalent bonds Figure 2.6-3 (slightly −) O H H (slightly +) (slightly +) Figure 2.6-3 Alternative ways to represent four common molecules (part 3) Polar covalent bonds in a water molecule

2.6 Covalent bonds join atoms into molecules through electron sharing H2 and O2 are molecules composed of only one element. Methane (CH4) and water (H2O) are compounds, substances composed of two or more different elements. Student Misconceptions and Concerns • Students with limited backgrounds in chemistry will benefit from a discussion of Figure 2.6 and the differences and limitations of representing atomic structure. The contrast in Figure 2.6 is a good beginning for such a discussion. In addition to comparing how the positions of electrons are depicted, note the problems with the sense of scale as discussed in Module 2.3. Teaching Tips • Have your students try to calculate the number of covalent bonds possible for a variety of atoms. (Carbon, for example, can form up to four covalent bonds.) Then provide the students with a list of elements and the number of outer electrons for each and have them make predictions about the chemical formula for many types of molecules. (For example, carbon could form covalent bonds with four hydrogen atoms.) • Modules 2.6 and 2.8 discuss the special bonding in and between water molecules. Many students do not appreciate the importance of weak chemical bonds in water and cellular chemistry. Extra time and attention may be required to address this special aspect of chemistry. Active Lecture Tips  Ask students to use 3 to 4 minutes to turn to someone near them to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Nearby pairs might provide immediate critiques with each other before passing along analogies for the entire class to consider. After these discussions, have pairs of students share their analogies with the entire class. Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and critical thinking skills. For example, ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both men look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) • See the essay Relating Chemical Bonds to Everyday Ideas on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

2.6 Covalent bonds join atoms into molecules through electron sharing Atoms in a covalently bonded molecule continually compete for shared electrons. The attraction (pull) for shared electrons is called electronegativity. More electronegative atoms pull harder. Student Misconceptions and Concerns • Students with limited backgrounds in chemistry will benefit from a discussion of Figure 2.6 and the differences and limitations of representing atomic structure. The contrast in Figure 2.6 is a good beginning for such a discussion. In addition to comparing how the positions of electrons are depicted, note the problems with the sense of scale as discussed in Module 2.3. Teaching Tips • Have your students try to calculate the number of covalent bonds possible for a variety of atoms. (Carbon, for example, can form up to four covalent bonds.) Then provide the students with a list of elements and the number of outer electrons for each and have them make predictions about the chemical formula for many types of molecules. (For example, carbon could form covalent bonds with four hydrogen atoms.) • Modules 2.6 and 2.8 discuss the special bonding in and between water molecules. Many students do not appreciate the importance of weak chemical bonds in water and cellular chemistry. Extra time and attention may be required to address this special aspect of chemistry. Active Lecture Tips  Ask students to use 3 to 4 minutes to turn to someone near them to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Nearby pairs might provide immediate critiques with each other before passing along analogies for the entire class to consider. After these discussions, have pairs of students share their analogies with the entire class. Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and critical thinking skills. For example, ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both men look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) • See the essay Relating Chemical Bonds to Everyday Ideas on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

2.6 Covalent bonds join atoms into molecules through electron sharing In molecules of only one element, the pull toward each atom is equal, because each atom has the same electronegativity. The bonds formed are called nonpolar covalent bonds. Student Misconceptions and Concerns • Students with limited backgrounds in chemistry will benefit from a discussion of Figure 2.6 and the differences and limitations of representing atomic structure. The contrast in Figure 2.6 is a good beginning for such a discussion. In addition to comparing how the positions of electrons are depicted, note the problems with the sense of scale as discussed in Module 2.3. Teaching Tips • Have your students try to calculate the number of covalent bonds possible for a variety of atoms. (Carbon, for example, can form up to four covalent bonds.) Then provide the students with a list of elements and the number of outer electrons for each and have them make predictions about the chemical formula for many types of molecules. (For example, carbon could form covalent bonds with four hydrogen atoms.) • Modules 2.6 and 2.8 discuss the special bonding in and between water molecules. Many students do not appreciate the importance of weak chemical bonds in water and cellular chemistry. Extra time and attention may be required to address this special aspect of chemistry. Active Lecture Tips  Ask students to use 3 to 4 minutes to turn to someone near them to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Nearby pairs might provide immediate critiques with each other before passing along analogies for the entire class to consider. After these discussions, have pairs of students share their analogies with the entire class. Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and critical thinking skills. For example, ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both men look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) • See the essay Relating Chemical Bonds to Everyday Ideas on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

2.6 Covalent bonds join atoms into molecules through electron sharing Water has atoms with different electronegativities. Oxygen attracts the shared electrons more strongly than hydrogen. So the shared electrons spend more time near oxygen. The oxygen atom has a slightly negative charge and the hydrogen atoms have a slightly positive charge. The result is a polar covalent bond. Student Misconceptions and Concerns • Students with limited backgrounds in chemistry will benefit from a discussion of Figure 2.6 and the differences and limitations of representing atomic structure. The contrast in Figure 2.6 is a good beginning for such a discussion. In addition to comparing how the positions of electrons are depicted, note the problems with the sense of scale as discussed in Module 2.3. Teaching Tips • Have your students try to calculate the number of covalent bonds possible for a variety of atoms. (Carbon, for example, can form up to four covalent bonds.) Then provide the students with a list of elements and the number of outer electrons for each and have them make predictions about the chemical formula for many types of molecules. (For example, carbon could form covalent bonds with four hydrogen atoms.) • Modules 2.6 and 2.8 discuss the special bonding in and between water molecules. Many students do not appreciate the importance of weak chemical bonds in water and cellular chemistry. Extra time and attention may be required to address this special aspect of chemistry. Active Lecture Tips  Ask students to use 3 to 4 minutes to turn to someone near them to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Nearby pairs might provide immediate critiques with each other before passing along analogies for the entire class to consider. After these discussions, have pairs of students share their analogies with the entire class. Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and critical thinking skills. For example, ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both men look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) • See the essay Relating Chemical Bonds to Everyday Ideas on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

2.7 Ionic bonds are attractions between ions of opposite charge An ion is an atom or molecule with an electrical charge resulting from gain or loss of one or more electrons. When an electron is lost, a positive charge results. When an electron is gained, a negative charge results. Two ions with opposite charges attract each other. When the attraction holds the ions together, it is called an ionic bond. Salt is a synonym for an ionic compound. Active Lecture Tips  Ask students to use 3 to 4 minutes to turn to someone near them to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Nearby pairs might provide immediate critiques with each other before passing along analogies for the entire class to consider. After these discussions, have pairs of students share their analogies with the entire class. Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and critical thinking skills. For example, ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both men look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) • See the essay Relating Chemical Bonds to Everyday Ideas on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

Na Cl Na Sodium atom Cl Chlorine atom Figure 2.7a-1 Na Cl Na Sodium atom Cl Chlorine atom Figure 2.7a-1 Formation of an ionic bond, producing sodium chloride (step 1)

Sodium chloride (NaCl) Figure 2.7a-2 − + Na Cl Na+ Cl− Na Sodium atom Cl Chlorine atom Na+ Sodium ion Cl− Chloride ion Figure 2.7a-2 Formation of an ionic bond, producing sodium chloride (step 2) Sodium chloride (NaCl)

Figure 2.7b-0 Cl− Na+ Figure 2.7b-0 A crystal of sodium chloride

2.8 Hydrogen bonds are weak bonds important in the chemistry of life In living organisms, most of the strong chemical bonds are covalent, linking atoms to form a cell’s molecules. Crucial to the functioning of a cell are weaker bonds within and between molecules. One of the most important types of weak bonds is the hydrogen bond, which is best illustrated with water molecules. Teaching Tips • Modules 2.6 and 2.8 discuss the special bonding in and between water molecules. Many students do not appreciate the importance of weak chemical bonds in water and cellular chemistry. Extra time and attention may be required to address this special aspect of chemistry. Active Lecture Tips  Ask students to use 3 to 4 minutes to turn to someone near them to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Nearby pairs might provide immediate critiques with each other before passing along analogies for the entire class to consider. After these discussions, have pairs of students share their analogies with the entire class. Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and critical thinking skills. For example, ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both men look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) • See the essay Relating Chemical Bonds to Everyday Ideas on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

2.8 Hydrogen bonds are weak bonds important in the chemistry of life The hydrogen atoms of a water molecule are attached to oxygen by polar covalent bonds. Because of these polar bonds and the wide V shape of the molecule, water is a polar molecule—that is, it has an unequal distribution of charges. This partial positive charge allows each hydrogen to be attracted to a nearby atom that has a partial negative charge. Teaching Tips • Modules 2.6 and 2.8 discuss the special bonding in and between water molecules. Many students do not appreciate the importance of weak chemical bonds in water and cellular chemistry. Extra time and attention may be required to address this special aspect of chemistry. Active Lecture Tips  Ask students to use 3 to 4 minutes to turn to someone near them to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Nearby pairs might provide immediate critiques with each other before passing along analogies for the entire class to consider. After these discussions, have pairs of students share their analogies with the entire class. Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and critical thinking skills. For example, ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both men look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) • See the essay Relating Chemical Bonds to Everyday Ideas on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

2.8 Hydrogen bonds are weak bonds important in the chemistry of life Weak hydrogen bonds form between water molecules. Each hydrogen atom of a water molecule can form a hydrogen bond with a nearby partially negative oxygen atom of another water molecule. The negative (oxygen) pole of a water molecule can form hydrogen bonds to two hydrogen atoms. Thus, each H2O molecule can hydrogen-bond to as many as four partners. Teaching Tips • Modules 2.6 and 2.8 discuss the special bonding in and between water molecules. Many students do not appreciate the importance of weak chemical bonds in water and cellular chemistry. Extra time and attention may be required to address this special aspect of chemistry. Active Lecture Tips  Ask students to use 3 to 4 minutes to turn to someone near them to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Nearby pairs might provide immediate critiques with each other before passing along analogies for the entire class to consider. After these discussions, have pairs of students share their analogies with the entire class. Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and critical thinking skills. For example, ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both men look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) • See the essay Relating Chemical Bonds to Everyday Ideas on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

(−) (+) (−) (+) (+) (−) (−) (+) Figure 2.8 (−) Hydrogen bond (+) Polar covalent bonds (−) (+) (+) (−) (−) Figure 2.8 Hydrogen bonds between water molecules (+)

2.9 Chemical reactions make and break chemical bonds Remember that the structure of atoms and molecules determines the way they behave. Atoms combine to form molecules. Hydrogen and oxygen can react to form water: 2 H2 + O2 2 H2O Student Misconceptions and Concerns • Students may misunderstand the chemical shorthand equation of photosynthesis presented in Module 2.9. As noted in the text, this overall equation does not include many smaller steps and reactions that occur in photosynthesis. If you discuss additional details of photosynthesis in your course, you might mention that you will add more to this equation at a later time. • A common student misconception is that energy is produced by a chemical reaction. When introducing chemical reactions, consider addressing the conservation of energy (the first law of thermodynamics) and the investment and release of energy in the creation and breaking of chemical bonds. Teaching Tips • As noted in the text, chemical reactions do not create or destroy matter. Instead, they rearrange the structure and form new relationships. This is much like shuffling and dealing cards. When playing poker, cards are neither created nor destroyed. Instead, new combinations are formed as the cards are dealt to the players. • The overall reaction of photosynthesis illustrates the investment and release of energy by chemical reactions. Consider discussing the investment of sunlight energy to create chemical bonds and the release of energy in the form of heat when plant materials are burned. (Animals invest some of the energy released by the breakdown of sugars to form new chemical bonds, such as those in ATP.) Active Lecture Tips  Ask students to use 3 to 4 minutes to turn to someone near them to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Nearby pairs might provide immediate critiques with each other before passing along analogies for the entire class to consider. After these discussions, have pairs of students share their analogies with the entire class. Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and critical thinking skills. For example, ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both men look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) • See the essay Relating Chemical Bonds to Everyday Ideas on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

2.9 Chemical reactions make and break chemical bonds The formation of water from hydrogen and oxygen is an example of a chemical reaction. The reactants (H2 and O2) are converted to H2O, the product. Chemical reactions do not create or destroy matter. Chemical reactions only rearrange matter. Student Misconceptions and Concerns • Students may misunderstand the chemical shorthand equation of photosynthesis presented in Module 2.9. As noted in the text, this overall equation does not include many smaller steps and reactions that occur in photosynthesis. If you discuss additional details of photosynthesis in your course, you might mention that you will add more to this equation at a later time. • A common student misconception is that energy is produced by a chemical reaction. When introducing chemical reactions, consider addressing the conservation of energy (the first law of thermodynamics) and the investment and release of energy in the creation and breaking of chemical bonds. Teaching Tips • As noted in the text, chemical reactions do not create or destroy matter. Instead, they rearrange the structure and form new relationships. This is much like shuffling and dealing cards. When playing poker, cards are neither created nor destroyed. Instead, new combinations are formed as the cards are dealt to the players. • The overall reaction of photosynthesis illustrates the investment and release of energy by chemical reactions. Consider discussing the investment of sunlight energy to create chemical bonds and the release of energy in the form of heat when plant materials are burned. (Animals invest some of the energy released by the breakdown of sugars to form new chemical bonds, such as those in ATP.) Active Lecture Tips  Ask students to use 3 to 4 minutes to turn to someone near them to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Nearby pairs might provide immediate critiques with each other before passing along analogies for the entire class to consider. After these discussions, have pairs of students share their analogies with the entire class. Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and critical thinking skills. For example, ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both men look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) • See the essay Relating Chemical Bonds to Everyday Ideas on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

2 H2 + O2 2 H2O Reactants Products Figure 2.9 Figure 2.9 Breaking and making of bonds in a chemical reaction

2.9 Chemical reactions make and break chemical bonds Photosynthesis is a chemical reaction that is essential to life on Earth. Carbon dioxide (from the air) reacts with water. Sunlight powers the conversion of these reactants to produce the products glucose and oxygen. Student Misconceptions and Concerns • Students may misunderstand the chemical shorthand equation of photosynthesis presented in Module 2.9. As noted in the text, this overall equation does not include many smaller steps and reactions that occur in photosynthesis. If you discuss additional details of photosynthesis in your course, you might mention that you will add more to this equation at a later time. • A common student misconception is that energy is produced by a chemical reaction. When introducing chemical reactions, consider addressing the conservation of energy (the first law of thermodynamics) and the investment and release of energy in the creation and breaking of chemical bonds. Teaching Tips • As noted in the text, chemical reactions do not create or destroy matter. Instead, they rearrange the structure and form new relationships. This is much like shuffling and dealing cards. When playing poker, cards are neither created nor destroyed. Instead, new combinations are formed as the cards are dealt to the players. • The overall reaction of photosynthesis illustrates the investment and release of energy by chemical reactions. Consider discussing the investment of sunlight energy to create chemical bonds and the release of energy in the form of heat when plant materials are burned. (Animals invest some of the energy released by the breakdown of sugars to form new chemical bonds, such as those in ATP.) Active Lecture Tips  Ask students to use 3 to 4 minutes to turn to someone near them to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Nearby pairs might provide immediate critiques with each other before passing along analogies for the entire class to consider. After these discussions, have pairs of students share their analogies with the entire class. Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and critical thinking skills. For example, ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both men look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) • See the essay Relating Chemical Bonds to Everyday Ideas on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

Water’s Life-Supporting Properties © 2015 Pearson Education, Inc.

2.10 Hydrogen bonds make liquid water cohesive The tendency of molecules of the same kind to stick together is cohesion. Cohesion is much stronger for water than for other liquids. Most plants depend upon cohesion to help transport water and nutrients from their roots to their leaves. The tendency of two kinds of molecules to stick together is adhesion. Student Misconceptions and Concerns • Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties may require high-level evaluative analysis. Teaching Tips • Here is a way to help your students think about the sticky nature of water in their lives. Ask them to consider the need for a towel after a shower or a bath. Once we get out of the shower or bath, we have left the source of water. So why do we need the towel? A towel helps us dry off water that is still clinging to our bodies because water molecules are polar. The molecules on cell surfaces are also polar, so our skin and the water stick to each other. • Some students may be intrigued if you tell them that you too can stand on the surface of water—when it is frozen. Thus, it is necessary to note a liquid water surface when discussing surface tension. © 2015 Pearson Education, Inc.

2.10 Hydrogen bonds make liquid water cohesive Cohesion is related to surface tension—a measure of how difficult it is to break the surface of a liquid. Hydrogen bonds give water high surface tension, making it behave as if it were coated with an invisible film. Water striders stand on water without breaking the water surface. Student Misconceptions and Concerns • Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties may require high-level evaluative analysis. Teaching Tips • Here is a way to help your students think about the sticky nature of water in their lives. Ask them to consider the need for a towel after a shower or a bath. Once we get out of the shower or bath, we have left the source of water. So why do we need the towel? A towel helps us dry off water that is still clinging to our bodies because water molecules are polar. The molecules on cell surfaces are also polar, so our skin and the water stick to each other. • Some students may be intrigued if you tell them that you too can stand on the surface of water—when it is frozen. Thus, it is necessary to note a liquid water surface when discussing surface tension. © 2015 Pearson Education, Inc.

Animation: Water Transport © 2015 Pearson Education, Inc.

Figure 2.10 Figure 2.10 Surface tension allowing a water strider to walk on water

2.11 Water’s hydrogen bonds moderate temperature Thermal energy is the energy associated with the random movement of atoms and molecules. Thermal energy in transfer from a warmer to a cooler body of matter is defined as heat. Temperature measures the intensity of heat—that is, the average speed of molecules in a body of matter. Student Misconceptions and Concerns • Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties may require high-level evaluative analysis. • Students at all levels struggle with the distinction between heat and temperature. Teaching Tips • Have students compare the seasonal ranges of temperatures of Anchorage and Fairbanks, Alaska. (Many websites, such as www.weather.com, provide weather information about various cities.) These two northern cities have large differences in their annual temperature ranges. Make the point that the coastal location of Anchorage moderates the temperature. • The following analogies may help students to understand the relationships between evaporation, heat, and temperature. (a) Ask students how the average on an exam would be affected if the brightest students did not take the test. (b) The authors note that the performance of a track team would drop if the fastest runners did not compete. In both analogies, removing the top performers lowers the average, just as the evaporation of the most active water molecules cools the evaporative surface. • It’s not the heat; it’s the humidity. The efficiency of evaporative cooling is affected by humidity. As humidity rises, the rate of evaporation decreases, making it more difficult to cool our heat-generating bodies on a warm and humid summer day. © 2015 Pearson Education, Inc.

2.11 Water’s hydrogen bonds moderate temperature Heat must be absorbed to break hydrogen bonds. Heat is released when hydrogen bonds form. To raise the temperature of water, hydrogen bonds between water molecules must be broken before the molecules can move faster. Thus, when warming up, water absorbs a large amount of heat and when water cools, water molecules slow down, more hydrogen bonds form, and a considerable amount of heat is released. Student Misconceptions and Concerns • Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties may require high-level evaluative analysis. • Students at all levels struggle with the distinction between heat and temperature. Teaching Tips • Have students compare the seasonal ranges of temperatures of Anchorage and Fairbanks, Alaska. (Many websites, such as www.weather.com, provide weather information about various cities.) These two northern cities have large differences in their annual temperature ranges. Make the point that the coastal location of Anchorage moderates the temperature. • The following analogies may help students to understand the relationships between evaporation, heat, and temperature. (a) Ask students how the average on an exam would be affected if the brightest students did not take the test. (b) The authors note that the performance of a track team would drop if the fastest runners did not compete. In both analogies, removing the top performers lowers the average, just as the evaporation of the most active water molecules cools the evaporative surface. • It’s not the heat; it’s the humidity. The efficiency of evaporative cooling is affected by humidity. As humidity rises, the rate of evaporation decreases, making it more difficult to cool our heat-generating bodies on a warm and humid summer day. © 2015 Pearson Education, Inc.

2.11 Water’s hydrogen bonds moderate temperature Earth’s giant water supply moderates temperatures, helping to keep temperatures within limits that permit life. Water’s resistance to temperature change also stabilizes ocean temperatures, creating a favorable environment for marine life. Student Misconceptions and Concerns • Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties may require high-level evaluative analysis. • Students at all levels struggle with the distinction between heat and temperature. Teaching Tips • Have students compare the seasonal ranges of temperatures of Anchorage and Fairbanks, Alaska. (Many websites, such as www.weather.com, provide weather information about various cities.) These two northern cities have large differences in their annual temperature ranges. Make the point that the coastal location of Anchorage moderates the temperature. • The following analogies may help students to understand the relationships between evaporation, heat, and temperature. (a) Ask students how the average on an exam would be affected if the brightest students did not take the test. (b) The authors note that the performance of a track team would drop if the fastest runners did not compete. In both analogies, removing the top performers lowers the average, just as the evaporation of the most active water molecules cools the evaporative surface. • It’s not the heat; it’s the humidity. The efficiency of evaporative cooling is affected by humidity. As humidity rises, the rate of evaporation decreases, making it more difficult to cool our heat-generating bodies on a warm and humid summer day. © 2015 Pearson Education, Inc.

2.11 Water’s hydrogen bonds moderate temperature When a substance evaporates, the surface of the liquid that remains behind cools down; this is the process of evaporative cooling. This cooling occurs because the molecules with the greatest energy leave the surface. Student Misconceptions and Concerns • Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties may require high-level evaluative analysis. • Students at all levels struggle with the distinction between heat and temperature. Teaching Tips • Have students compare the seasonal ranges of temperatures of Anchorage and Fairbanks, Alaska. (Many websites, such as www.weather.com, provide weather information about various cities.) These two northern cities have large differences in their annual temperature ranges. Make the point that the coastal location of Anchorage moderates the temperature. • The following analogies may help students to understand the relationships between evaporation, heat, and temperature. (a) Ask students how the average on an exam would be affected if the brightest students did not take the test. (b) The authors note that the performance of a track team would drop if the fastest runners did not compete. In both analogies, removing the top performers lowers the average, just as the evaporation of the most active water molecules cools the evaporative surface. • It’s not the heat, it’s the humidity. The efficiency of evaporative cooling is affected by humidity. As humidity rises, the rate of evaporation decreases, making it more difficult to cool our heat-generating bodies on a warm and humid summer day. © 2015 Pearson Education, Inc.

2.12 Ice floats because it is less dense than liquid water Water can exist as a gas, liquid, or solid. Water is less dense as a solid than a liquid because of hydrogen bonding. When water freezes, each molecule forms a stable hydrogen bond with its neighbors. As ice crystals form, the molecules are less densely packed than in liquid water. Because ice is less dense than water, it floats. Student Misconceptions and Concerns • Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties may require high-level evaluative analysis. • Students might also expect that all ice is about the same temperature, 0ºC. Redefining and correcting misunderstandings often takes more class time and energy than introducing previously unknown concepts. Teaching Tips • Ask your students if the ocean levels would change if ice did not float. They can try this experiment to find out, or you can begin class with the demonstration and watch the progress throughout the class period. Place several large chunks of ice in a glass and fill the glass up completely with water to the top rim. Thus, the ice cubes should be sticking up above the top of the filled glass. Will the glass overflow when the ice melts? (No.) This phenomenon is important when we consider the potential consequences of global warming. If floating glaciers melt, ocean levels will not be affected. However, if the ice over land melts, we can expect higher ocean levels. Active Lecture Tips • Module 2.12 notes the insulating effect of ice forming at the surface of a lake. This phenomenon would not occur if ice were denser than water. Ask students to turn to someone near them to think of other consequences from the expansion of water when it forms ice. After perhaps 2 minutes, have pairs of students contribute the examples they came up with for a quick discussion. (These include the ability to widen cracks in rocks, roads, and sidewalks!) © 2015 Pearson Education, Inc.

Ice Hydrogen bonds are stable. Figure 2.12-0 Ice Hydrogen bonds are stable. Hydrogen bond Figure 2.12-0 Hydrogen bonds between water molecules in ice and water Liquid water Hydrogen bonds constantly break and re-form.

2.13 Water is the solvent of life A solution is a liquid consisting of a uniform mixture of two or more substances. The dissolving agent is the solvent. The substance that is dissolved is the solute. An aqueous solution is one in which water is the solvent. Student Misconceptions and Concerns • Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties may require high-level evaluative analysis. Teaching Tips • A simple demonstration of a solute dissolving in a solvent can focus students’ attention on the process when discussing solutions. Using colored water and white sugar or salt may make it easier to see and reference while you are discussing the process. Such simple visual aids add life to a lecture. (You might also add corn oil to the top of the solution to demonstrate the properties of hydrophobic substances, and challenge your class to explain why oil and water do not mix.) © 2015 Pearson Education, Inc.

2.13 Water is the solvent of life Water’s versatility as a solvent results from the polarity of its molecules. Polar or charged solutes dissolve when water molecules surround them, forming aqueous solutions. Table salt is an example of a solute that will go into solution in water. Student Misconceptions and Concerns • Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties may require high-level evaluative analysis. Teaching Tips • A simple demonstration of a solute dissolving in a solvent can focus students’ attention on the process when discussing solutions. Using colored water and white sugar or salt may make it easier to see and reference while you are discussing the process. Such simple visual aids add life to a lecture. (You might also add corn oil to the top of the solution to demonstrate the properties of hydrophobic substances, and challenge your class to explain why oil and water do not mix.) © 2015 Pearson Education, Inc.

Figure 2.13 Positive hydrogen ends of water molecules attracted to negative chloride ion Negative oxygen ends of water molecules attracted to positive sodium ion Na+ Cl− + − + − Cl− − + Na+ − + Figure 2.13 A crystal of salt (NaCl) dissolving in water + + − − + − − − + Salt crystal

2.14 The chemistry of life is sensitive to acidic and basic conditions In liquid water, a small percentage of water molecules break apart into ions. Some are hydrogen ions (H+). Some are hydroxide ions (OH–). Both types are very reactive. Teaching Tips • Discussions of pH are enhanced by lab activities that permit students to test the pH of everyday items (foods and household solutions). If students do not have opportunities to conduct such tests in lab, consider testing a few items during your class (pH paper or a basic pH meter will, of course, be necessary). Active Lecture Tips • See the essay The Earthquake Richter Scale and the pH Scale on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

2.14 The chemistry of life is sensitive to acidic and basic conditions A substance that donates hydrogen ions to solutions is called an acid. A base is a substance that reduces the hydrogen ion concentration of a solution. The pH scale describes how acidic or basic a solution is. The pH scale ranges from 0 to 14, with 0 the most acidic and 14 the most basic. Each pH unit represents a 10-fold change in the concentration of H+ in a solution. Teaching Tips • Discussions of pH are enhanced by lab activities that permit students to test the pH of everyday items (foods and household solutions). If students do not have opportunities to conduct such tests in lab, consider testing a few items during your class (pH paper or a basic pH meter will, of course, be necessary). Active Lecture Tips • See the essay The Earthquake Richter Scale and the pH Scale on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

2.14 The chemistry of life is sensitive to acidic and basic conditions A buffer is a substance that minimizes changes in pH. Buffers accept H+ when it is in excess and donate H+ when it is depleted. Teaching Tips • Discussions of pH are enhanced by lab activities that permit students to test the pH of everyday items (foods and household solutions). If students do not have opportunities to conduct such tests in lab, consider testing a few items during your class (pH paper or a basic pH meter will, of course, be necessary). Active Lecture Tips • See the essay The Earthquake Richter Scale and the pH Scale on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. © 2015 Pearson Education, Inc.

Lemon juice, gastric juice Figure 2.14-0 pH scale Battery acid H+ H+ 1 H+ OH− H+ OH− H+ 2 Lemon juice, gastric juice H+ H+ H+ 3 Vinegar, cola Acidic solution Increasingly ACIDIC (Higher H+ concentration) 4 Tomato juice 5 Rainwater 6 Human urine Saliva H+ OH− OH− NEUTRAL [H+] = [OH−] H+ OH− OH− 7 Pure water Human blood, tears OH− H+ H+ H+ 8 Seawater Neutral solution 9 Figure 2.14-0 The pH scale, which reflects the relative concentrations of H+ and OH− 10 Increasingly BASIC (Higher OH− concentration) Milk of magnesia OH− OH− 11 OH− H+ OH− Household ammonia OH− 12 OH− OH− H+ Household bleach 13 Basic solution Oven cleaner 14

Acidic solution Neutral solution Basic solution Figure 2.14-3 H+ H+ H+ OH− OH− OH− OH− H+ OH− OH− OH− H+ OH− H+ H+ OH− OH− H+ OH− OH− H+ H+ H+ H+ OH− OH− H+ H+ H+ Acidic solution Neutral solution Basic solution Figure 2.14-3 The pH scale, which reflects the relative concentrations of H+ and OH− (part 3)

Protons (+ charge) determine element Nucleus Figure 2.UN01 Protons (+ charge) determine element Nucleus Electrons (− charge) form negative cloud and determine chemical behavior − + + − Neutrons (no charge) determine isotope Atom Figure 2.UN01 Reviewing the concepts, 2.3

Liquid water: Hydrogen bonds constantly break and re-form Figure 2.UN02 Liquid water: Hydrogen bonds constantly break and re-form Ice: Stable hydrogen bonds hold molecules apart Figure 2.UN02 Reviewing the concepts, 2.12

F K Fluorine atom Potassium atom Figure 2.UN04 F K Fluorine atom Potassium atom Figure 2.UN04 Testing your knowledge, question 11