Atoms, Molecules, and Life

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Presentation transcript:

Atoms, Molecules, and Life 2 Atoms, Molecules, and Life 1

Chapter 2 At a Glance 2.1 What Are Atoms? 2.2 How Do Atoms Interact to Form Molecules? 2.3 Why Is Water So Important to Life?

2.1 What Are Atoms? Atoms are the basic structural units of elements Elements are substances that cannot be broken down into simpler substances Elements cannot be transformed into other substances under ordinary conditions Elements form all matter alone and when combined with other elements An atom is the smallest unit of an element, and retains all the chemical properties of that element All atoms belong to one of the 98 naturally occurring elements. 118 total as of 2017.

Table 2-1 4

2.1 What Are Atoms? Atoms are composed of subatomic particles Atoms are composed of still-smaller particles Atoms are composed of subatomic particles In the central atomic nucleus, there are positively charged protons and uncharged neutrons In orbit around the nucleus are negatively charged particles called electrons Atoms are electrically neutral because their number of positive protons and negative electrons is equal

Table 2-2 6

2.1 What Are Atoms? Atoms are composed of still-smaller particles (continued) Subatomic particles are measured in atomic mass because they are very small The mass number of an atom is the total mass of the protons and neutrons contained in the nucleus The atomic nucleus is formed by the cluster of protons and neutrons in the center of the atom Electrons rapidly orbit around the nucleus in a continuous motion

2.1 What Are Atoms? Elements are defined by their atomic numbers The number of protons in the nucleus of an atom is known as the atom’s atomic number For example, hydrogen atoms have one proton, so the atomic number for hydrogen is one The atomic number is the defining value of each element For example, carbon has six protons and oxygen has eight

2.1 What Are Atoms? Isotopes are atoms of the same element with different numbers of neutrons Variant atomic forms of an element are called isotopes Some isotopes are radioactive (meaning that they spontaneously break apart, forming different atoms and releasing energy) and are used in research At room temperature, elements may occur as solids, liquids, or gases

Hydrogen (H) Helium (He) e electron e shell p p p n n atomic Figure 2-1 Atomic models e e electron shell p p p n n atomic nucleus e Hydrogen (H) Helium (He) 11

2.1 What Are Atoms? Nuclei and electrons play complementary roles in atoms Nuclei provide stability by resisting external forces such as energy, heat, and electricity For example, the stable nuclei of a carbon atom (12C) keeps its carbonic identity regardless of the different structures it may be found in Electrons are dynamic They can capture and release energy that forms bonds used to combine atoms

2.1 What Are Atoms? Electrons occupy complex regions around the nucleus Electrons are distributed around the nucleus of an atom in electron shells

Figure 2-2 Electron shells in atoms 6p 8p 15p 20p 6n 8n 16n 20n Carbon (C) Oxygen (O) Phosphorus (P) Calcium (Ca) 14

2.1 What Are Atoms? Electrons can capture and release energy When energy excites an atom, it causes an electron to jump from a lower- to a higher-energy shell Later, the electron falls back into its original shell, releasing the energy Life depends on electrons capturing and releasing energy For example, the use of the light bulb

Figure 2-3 Energy capture and release The energy boosts the electron to a higher-energy shell The electron drops back into lower-energy shell, releasing energy as light An electron absorbs energy heat energy light 16

2.1 What Are Atoms? As atomic number increases, electrons fill shells increasingly distant from the nucleus Each electron shell holds a specific number of electrons The first shell, or energy level, holds two electrons The second shell holds up to eight Electrons fill the shell closest to the nucleus first in an effort to maintain stability

2.2 How Do Atoms Interact to Form Molecules? Matter consists of atoms Molecules consist of two or more atoms, from the same or different elements, that are held together by interactions among their outermost electron shells For example, hydrogen and oxygen are linked to form water (H2O is two atoms of hydrogen and one atom of oxygen) Table salt is another example (NaCl is one atom of sodium and one atom of chloride)

2.2 How Do Atoms Interact to Form Molecules? Atoms form molecules to fill vacancies in their outer electron shells Atoms will not react with other atoms if the outermost shell is completely empty or full (such atoms are considered inert) Example: Neon, with eight electrons in the outermost shell, is full and, therefore, inert

2.2 How Do Atoms Interact to Form Molecules? Atoms form molecules to fill vacancies in their outer electron shells (continued) Atoms will react with other atoms if the outermost shell is partially full (such atoms are considered reactive) Example: Oxygen, with six electrons in its outermost shell, can hold two more electrons, and so is susceptible to reacting

2.2 How Do Atoms Interact to Form Molecules? Chemical bonds hold atoms together in molecules There are three types of chemical bonds, which are the attractive forces holding atoms together in molecules: ionic bond, covalent bond, and hydrogen bond Reactive atoms gain stability through electron interactions (chemical bonds) Hydrogen and oxygen atoms gain stability by interacting with each other Single electrons from each of two hydrogen molecules fill the outer shell of an oxygen atom

Table 2-3 22

2.2 How Do Atoms Interact to Form Molecules? Ionic bonds form among ions Atoms that have lost or gained electrons, thereby altering the balance between protons and electrons, are charged, and are called ions Atoms that have lost electrons become positively charged ions (e.g., sodium: Na) Atoms that have gained electrons become negatively charged ions (e.g., chlorine: Cl) Oppositely charged ions that are attracted to each other are bound into a molecule by ionic bonds

Figure 2-4 The formation of ions and ionic bonds Sodium atom (neutral) Chlorine atom (neutral) 11p 17p 11n 18n Electron transferred Neutral atoms Sodium ion () Chloride ion () 11p 17p 11n 18n Attraction between opposite charges Ions Cl Na Cl Na Cl Na Cl Na Cl An ionic compound: NaCl 24

2.2 How Do Atoms Interact to Form Molecules? Covalent bonds form by sharing electrons An atom with a partially full outermost electron shell can become stable by sharing electrons with another atom, forming a covalent bond Two electrons (one from each atom), when shared, form a covalent bond

2.2 How Do Atoms Interact to Form Molecules? Covalent bonds form by sharing electrons (continued) Covalent bonds are found in H2 (single bond), O2 (double bond), N2 (triple bond), and H2O Covalent bonds are stronger than ionic bonds but vary in their stability

2.2 How Do Atoms Interact to Form Molecules? Covalent bonds form by sharing electrons (continued) Because biological molecules must function in a watery environment where ionic bonds rapidly dissociate (break down), the atoms in most biological molecules, such as those found in proteins, sugars, and fats, are joined by covalent bonds

Table 2-4 28

2.2 How Do Atoms Interact to Form Molecules? Covalent bonds may produce nonpolar or polar molecules Atoms within a molecule may have different nuclear charges Those atoms with a greater positive nuclear charge pull more strongly on electrons in a covalent bond

2.2 How Do Atoms Interact to Form Molecules? Covalent bonds may produce nonpolar or polar molecules (continued) In molecules such as H2, both atoms exert the same pulling force on bond electrons and the bond is called a nonpolar covalent bond In molecules where atoms of different elements are involved (H2O), the electrons are not always equally shared and these covalent bonds are called polar covalent bonds

2.2 How Do Atoms Interact to Form Molecules? Covalent bonds may produce nonpolar or polar molecules (continued) A molecule with polar bonds may be polar overall H2O is a polar molecule The (slightly) positively charged pole is around each hydrogen The (slightly) negatively charged pole is around the oxygen

Figure 2-5 Covalent bonds involve shared electrons (oxygen: slightly negative) () Larger positive charge Same charge on both nuclei Electrons spend more time near the larger nucleus 8p 8n Electrons spend equal time near each nucleus Smaller positive charge (hydrogens: slightly positive) (hydrogens: uncharged) () () Nonpolar covalent bonding in hydrogen gas (H2) Polar covalent bonding in water (H2O) 32

2.2 How Do Atoms Interact to Form Molecules? Hydrogen bonds are attractive forces between polar molecules Polar molecules—such as those in water—have partially charged atoms at their ends Hydrogen bonds form when partial opposite charges in different molecules attract each other The partially positive hydrogen atoms of one water molecule are attracted to the partially negative oxygen on another

2.2 How Do Atoms Interact to Form Molecules? Hydrogen bonds are attractive forces between polar molecules (continued) Polar biological molecules can form hydrogen bonds with water, each other, or even within the same molecule Hydrogen bonds are comparatively weak but, collectively, can be quite strong

Figure 2-6 Hydrogen bonds in water () () H () H () O () H () hydrogen bonds 35

2.3 Why Is Water So Important to Life? Water molecules attract one another Cohesion is the tendency of the molecules of a substance to stick together Hydrogen bonding between water molecules produces high cohesion Water cohesion explains how water molecules can form a chain in delivering moisture to the top of a tree

2.3 Why Is Water So Important to Life? Water molecules attract one another (continued) Cohesion of water molecules along a surface produces surface tension Spiders and water striders rely on surface tension to move across the surface of ponds

Figure 2-7 Cohesion among water molecules Cohesion helps water to reach treetops Cohesion causes surface tension 38

2.3 Why Is Water So Important to Life? Water interacts with many other molecules Water is an excellent solvent A wide range of substances dissolve (are completely surrounded and dispersed) in water to form solutions

Figure 2-8 Water as a solvent Cl Na H Na Cl O H Cl Na 40

2.3 Why Is Water So Important to Life? Water interacts with many other molecules (continued) Water-soluble molecules are hydrophilic Water molecules are attracted to and can surround (and thereby dissolve) ions or polar molecules, such as sugars and amino acids

2.3 Why Is Water So Important to Life? Water interacts with many other molecules (continued) Water-insoluble molecules that repel and drive together uncharged and nonpolar molecules such as fats and oils are hydrophobic The “clumping” of nonpolar molecules is called hydrophobic interaction

Figure 2-9 Oil and water don’t mix 43

2.3 Why Is Water So Important to Life? Water moderates the effects of temperature changes Very low or very high temperatures may damage enzymes or slow down important chemical reactions A lot of energy is required to heat water The evaporation of water also requires a lot of energy

2.3 Why Is Water So Important to Life? Water moderates the effects of temperature changes (continued) It takes a lot of energy to heat water The energy required to heat 1 gram of a substance by 1°C is called its specific heat Temperature reflects the speed of molecular motion It requires 1 calorie of energy to raise the temperature of 1g of water 1°C (the specific heat of water), which is a very slow process

2.3 Why Is Water So Important to Life? Water moderates the effects of temperature changes (continued) It takes a lot of energy to evaporate water The heat of vaporization is the amount of heat needed to cause a substance such as water to evaporate (to change from a liquid to a vapor) Evaporating water uses up heat from its surroundings, cooling the nearby environment (as occurs during sweating) Because the human body is mostly water, a sunbather can absorb a lot of heat energy without sending her/his body temperature soaring

2.3 Why Is Water So Important to Life? Water forms an unusual solid: ice Most substances become denser when they solidify from a liquid Ice is unusual because it is less dense than liquid water Water molecules spread apart slightly during the freezing process

Figure 2-11 Liquid water and ice 48

2.3 Why Is Water So Important to Life? Water forms an unusual solid: ice (continued) Ice floats in liquid water Ponds and lakes freeze from the top down and never freeze completely to the bottom Many plants and fish are therefore saved from freezing

Figure 2-12 Ice floats 50

2.3 Why Is Water So Important to Life? Water-based solutions can be acidic, basic, or neutral A small fraction of water molecules break apart into ions H2O  OH  H Solutions in which H  OH are acidic For example, hydrochloric acid ionizes in water HCl  H  Cl Lemon juice and vinegar are naturally occurring acids

2.3 Why Is Water So Important to Life? Water-based solutions can be acidic, basic, or neutral (continued) Solutions in which OH  H are basic For example, sodium hydroxide ionizes in water NaOH  Na  OH Baking soda, chlorine bleach, and ammonia are basic

Figure 2-13 Some water is always ionized () () O O H H H H water hydroxide ion hydrogen ion (H2O) (OH) (H) 53

2.3 Why Is Water So Important to Life? Water-based solutions can be acidic, basic, or neutral (continued) The degree of acidity of a solution is measured using the pH scale pH 0–6 is acidic (H  OH) pH 7 is neutral (H  OH) pH 8–14 is basic (OH  H)

H concentration in moles/liter Figure 2-14 The pH scale household ammonia (11.9) 1 molar hydrochloric acid (HCI) “acid rain” (2.5–5.5) chlorine bleach (12.6) seawater (7.8–8.3) drain cleaner (14.0) hydroxide (NaOH) 1 molar sodium stomach acid (2) lemon juice (2.3) vinegar, cola (3.0) black coffee (5.0) blood, sweat (7.4) baking soda (8.4) washing soda (12) oven cleaner (13.0) normal rain (5.6) pure water (7.0) tomatoes (4.5) orange (3.5) antacid (10) beer (4.1) urine (5.7) milk (6.4) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH value (H  OH) (H  OH) neutral increasingly acidic increasingly basic (H  OH) 100 101 102 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 H concentration in moles/liter 55

2.3 Why Is Water So Important to Life? Water-based solutions can be acidic, basic, or neutral (continued) A buffer is a type of molecule that helps a solution maintain constant pH Buffers accept or release H in response to a pH change The bicarbonate buffer found in our bloodstream prevents pH changes

2.3 Why Is Water So Important to Life? Water-based solutions can be acidic, basic, or neutral (continued) If the blood becomes too acidic, bicarbonate accepts (and absorbs) H to make carbonic acid HCO3  H  H2CO3 bicarbonate hydrogen ion carbonic acid

2.3 Why Is Water So Important to Life? Water-based solutions can be acidic, basic, or neutral (continued) If the blood becomes too basic, carbonic acid liberates hydrogen ions to combine with OH to form water H2CO3  OH  HCO3  H2O carbonic acid hydroxide ion bicarbonate water