Fundamental Building Blocks: Chemistry, Water, and pH

2.1 Chemistry’s Building Block: The Atom

Chemistry’s Building Block: The Atom The fundamental unit of matter is the atom.

Protons, Neurons, and Electrons The three most important constituent parts of an atom are protons neutrons electrons

Protons, Neurons, and Electrons Protons and neutrons exist in the atom’s nucleus, while electrons move around the nucleus, at some distance from it.

Chapter 2 Lecture electron (negative charge) electron shell proton (positive charge) nucleus neutron (no charge) Hydrogen (H) Helium (He) Figure 2.2

Protons, Neurons, and Electrons Protons are positively charged. Electrons are negatively charged. Neutrons carry no charge.

The Element An element is any substance that cannot be reduced to any simpler set of constituent substances through chemical means. Each element is defined by the number of protons in its nucleus.

The Element The number of neutrons in an atom can vary independently of the number of protons. Thus, a single element can exist in various forms, called isotopes, depending on the number of neutrons it possesses.

The Element Hydrogen Deuterium Tritium 1 proton 0 neutrons 1 proton Figure 2.5

The Element PLAY PLAY PLAY Animation 2.1: Structure of Atoms, Elements, Isotopes: Part 1: Animation PLAY Animation 2.1: Structure of Atoms, Elements, Isotopes: Part 2: Exercise 1 PLAY Animation 2.1: Structure of Atoms, Elements, Isotopes: Part 3: Exercise 2

The Element Protons are positively charged. Electrons are negatively charged. Neutrons carry no charge.

2.2 Matter is Transformed Through Chemical Bonding

Matter is Transformed Through Chemical Bonding Atoms can link to one another in the process of chemical bonding.

Covalent Bond Covalent bond: atoms share one or more electrons Ionic bond: atoms lose and accept electrons from each other

Covalent Bond Chemical bonding comes about as atoms “seek” their lowest energy state. An atom achieves this state when it has a filled outer electron shell.

Covalent Bond Hydrogen and helium require two electrons in orbit around their nuclei to have filled outer shells. Most other elements require eight electrons to have filled outer shells.

Covalent Bond Figure 2.7 Unstable, very reactive atoms Stable, unreactive atoms hydrogen (H) helium (He) carbon (C) neon (Ne) sodium (Na) argon (Ar) Outermost electron shells unfilled Outermost electron shells filled Figure 2.7

Covalent Bond A molecule is a compound of a defined number of atoms in a defined spatial relationship. For example, two hydrogen atoms can link with one oxygen atom to form one water molecule.

Covalent Bond (a) Two hydrogen atoms and one oxygen atom hydrogen (H) atom hydrogen (H) atom hydrogen (H) atom hydrogen (H) atom oxygen (O) atom oxygen (O) atom (a) Two hydrogen atoms and one oxygen atom (b) One water molecule Figure 2.8

Covalent Bond Atoms of different elements differ in their power to attract electrons. The term for measuring this power is electronegativity.

Polar and Nonpolar Bonding Through electronegativity, a molecule can take on a polarity—a difference in electrical charge at one end compared to the other.

Polar and Nonpolar Bonding Covalent chemical bonds can be polar or nonpolar. A polar covalent bond exists when shared electrons are not shared equally among atoms in a molecule, due to electronegativity differences.

Polar and Nonpolar Bonding (a) Polar water molecule (b) Nonpolar methane molecule slight negative charge polar nonpolar because charges are symmetric slight positive charge Figure 2.9

Ionic Bonding Two atoms will undergo a process of ionization when the electronegativity differences between them are great enough that one atom loses one or more electrons to the other. This process creates ions: atoms whose number of electrons differs from their number of protons.

Ionic Bonding The charge differences that result from ionization can produce an electrostatic attraction between ions. This attraction is an ionic bond. When atoms of two or more elements bond together ionically, the result is an ionic compound.

Ionic Bonding Figure 2.10 sodium atom (Na) chlorine atom (Cl) (a) Initial instability Sodium has but a single electron in its outer shell, while chlorine has seven, meaning it lacks only a single electron to have a completed outer shell. electron transfer (b) Electron transfer When these two atoms come together, sodium loses its third-shell electron to chlorine, in the process becoming a sodium ion with a net positive charge (because it now has more protons than electrons). Having gained an electron, the chlorine atom becomes a chloride ion, with a net negative charge (because it has more electrons than protons). chloride ion (Cl–) sodium ion (Na+) ionic compound (Na+Cl–) (c) Ionic attraction The sodium and chloride ions are now attracted to each other because they are oppositely charged. (d) Compound formation The result of this electrostatic attraction, involving many sodium and chloride ions, is a sodium chloride crystal (NaCl), better known as table salt. salt crystals Figure 2.10

Hydrogen Bonding Hydrogen bonding links an already covalently bonded hydrogen atom with an electronegative atom.

Hydrogen Bonding In water, a hydrogen atom of one water molecule will form a hydrogen bond with an unshared oxygen electron of a neighboring water molecule.

Hydrogen Bonding hydrogen bond Figure 2.11

Hydrogen Bonding PLAY Animation 2.2: Chemical Bonding

2.3 Some Qualities of Chemical Compounds

Some Qualities of Chemical Compounds The three-dimensional molecular shape is important in biology because it determines the capacity molecules have to bind with one another.

Molecular Shape good fit, scent is smelled signal to brain signal molecules (aroma from bread) receptor molecules bad fit, scent is not smelled cells of nasal passage Figure 2.13

Molecular Shape PLAY Animation 2.3: Geometry, Chemistry, and Biology

2.4 Water and Life

Water and Life Water has several qualities that have strongly affected life on Earth.

Water is a Major Player in Many of Life’s Processes A solution is a homogeneous mixture of two or more kinds of molecules, atoms, or ions. The compound dissolved in solution is the solute; the compound doing the dissolving is the solvent.

Water is a Major Player in Many of Life’s Processes (a) Attraction (b) Separation (c) Dispersion water (solvent) Sodium and chloride ions dissolved in water H O H Na+ Cl– sodium chloride (solute) Sodium chloride’s positively charged sodium ions (Na+) are attracted to water’s negatively charged oxygen atoms, while its negatively charged chloride ions (Cl–) are attracted to water’s positively charged hydrogen atoms. Pulled from the crystal, and separated from each other by this attraction, sodium and chloride ions become surrounded by water molecules. This process of separating sodium and chloride ions repeats until both ions are evenly dispersed, making this an aqueous solution. Figure 2.15

Water is a Major Player in Many of Life’s Processes Water is a powerful solvent, with the ability to dissolve more compounds in greater amounts than any other liquid.

Water’s Structure Gives It Many Unusual Properties Because water’s solid form (ice) is less dense than its liquid form, bodies of water in colder climates do not freeze solid in winter. Allows life to flourish under the ice.

Water’s Structure Gives It Many Unusual Properties ice In ice, the maximum number of hydrogen bonds form, causing the molecules to be spread far apart. liquid water In liquid water, hydrogen bonds constantly break and re-form, enabling a more dense spacing than in ice. Figure 2.16

Water’s Structure Gives It Many Unusual Properties Water has a great capacity to absorb and retain heat. Because of this, the oceans act as heat buffers for the Earth, thus stabilizing Earth’s temperature.

Water’s Structure Gives It Many Unusual Properties Water has a high degree of cohesion, which allows water to be drawn up through plants, via evaporation, in one continuous column, from roots through leaves.

Hydrophobic and Hydrophilic Some compounds do not interact with water. Hydrocarbons such as petroleum are examples of such hydrophobic compounds. Water cannot break down hydrophobic compounds, which is why oil and water don’t mix.

Hydrophobic and Hydrophilic Figure 2.17

Hydrophobic and Hydrophilic Compounds that interact with water are polar or carry an electric charge and are called hydrophilic compounds.

2.5 Acids and Bases Are Important to Life

Acids and Bases Are Important to Life An acid is any substance that yields hydrogen ions when put in aqueous solution. A base is any substance that accepts hydrogen ions in solution.

Acids and Bases A base added to an acidic solution makes that solution less acidic. An acid added to a basic solution makes that solution less basic.

Acids and Bases Figure 2.18 pure water (a) Starting with pure water Pure water is a “neutral” substance in terms of its pH levels. (H2O) (b) Making water more acidic (c) Making water more basic HCl NaOH Hydrochloric acid (HCl), poured into the water, dissociates into H+ and Cl- ions. With a higher concentration of H+ ions in it, the water moves towards the acidic end of the pH scale. An equal concentration of sodium hydroxide, poured into water, dissociates into Na+ and OH– ions, moving the water toward the basic end of the scale. (d) Combining acidic and basic solutions When the acid and base solutions are poured together, the OH– ions from (c) accept the H+ ions from (b), forming water and keeping the solution at a neutral pH. acid base neutralized solution Figure 2.18

Acids and Bases The concentration of hydrogen ions that a given solution has determines how basic or acidic that solution is. The pH scale measures acidity. This scale runs from 0 to 14, with 0 most acidic,14 the most basic, and 7 neutral.

Acids and Bases Figure 2.19 H+ concentration (moles/liter) pH acidic battery acid hydrochloric acid lemon juice, gastric (stomach) juice cola, beer, wine, vinegar tomatoes black coffee urine neutral water human blood seawater human blood is slightly basic baking soda Great Salt Lake household ammonia household bleach oven cleaner lye basic Figure 2.19

Acids and Bases The pH scale is logarithmic. A substance with a pH of 9 is 10 times as basic as a substance with a pH of 8

Acids and Bases Living things function best in a near-neutral pH, although some systems in living things have different pH requirements.

Acids and Bases PLAY Animation 2.4: Water and pH