1. Chapter 2 Life’s Chemical Basis (Sections 2.4 - 2.6)

2. 2.4 Why Atoms Interact
The characteristics of a bond arise from the properties of atoms that take part in it
Atoms form different types of bonds depending on their electronegativity
Ionic bonds
Covalent bonds
Hydrogen bonds

3. Ionic Bonds
An ionic bond is a strong association between oppositely charged ions that arises from the mutual attraction of opposite charges
ionic bond
Type of chemical bond in which a strong mutual attraction forms between ions of opposite charge

4. A Each crystal of table salt is a cubic lattice of many sodium and chloride ions locked in ionic bonds.
ionic bond
Figure 2.7 Ionic bonds. 11 17
Sodium ion
11p+, 10e–
Chloride ion
17p+, 18e–
B The mutual attraction of opposite charges holds the two kinds of ions together in the lattice.
Fig. 2.7, p. 28

5. Figure 2.7 Ionic bonds.
A Each crystal of table salt is a cubic lattice of many sodium and chloride ions locked in ionic bonds.
Fig. 2.7a, p. 28

6. ionic bond 11 17 Sodium ion 11p+, 10e– Chloride ion 17p+, 18e–
Figure 2.7 Ionic bonds.
Sodium ion
11p+, 10e–
Chloride ion
17p+, 18e–
B The mutual attraction of opposite charges holds the two kinds of ions together in the lattice.
Fig. 2.7b, p. 28

7. Covalent Bonds
Atoms share a pair of electrons in a covalent bond, which is nonpolar if the sharing is equal, and polar if it is not
covalent bond
Chemical bond in which two atoms share a pair of electrons
polarity
Any separation of charge into distinct positive and negative regions

8. Multiple Covalent Bonds
Many atoms participate in more than one covalent bond
Two, three, or four covalent bonds may form between two atoms when they share multiple electrons
Molecular oxygen: O=O
Molecular nitrogen: (N=N)

9. Covalent Bonds

10. Molecular hydrogen (H H)
Two hydrogen atoms, each with one proton, share two electrons in a single nonpolar covalent bond. 1 1
Molecular oxygen (O O)
Two oxygen atoms, each with eight protons, share four electrons in a double covalent bond. 8 8
Water molecule (H O H)
Figure 2.8 Covalent bonds, in which atoms with unpaired electrons in their outermost shell become more stable by sharing electrons. Two electrons are shared in each covalent bond. When sharing is equal, the bond is nonpolar. When one atom exerts a greater pull on the electrons, the bond is polar.
Two hydrogen atoms share electrons with an oxygen atom in two polar covalent bonds. The oxygen exerts a greater pull on the shared electrons, so it has a slight negative charge. Each hydrogen has a slight positive charge. 8 1 1
Fig. 2.8, p. 29

11. Ways of Representing a Molecule

12. Hydrogen Bonds
Hydrogen bonds collectively stabilize the structures of large molecules
hydrogen bond
Attraction that forms between a covalently bonded hydrogen atom and another atom taking part in a separate covalent bond

13. Hydrogen Bonds in Water
Hydrogen bonds form between opposite charges in polar water molecules
An important property for life

14. hydrogen bond H H O H O H water molecule water molecule
Figure 2.9 Hydrogen bonds. Hydrogen bonds form at a hydrogen atom taking part in a polar covalent bond. The hydrogen atom’s slight positive charge weakly attracts an electronegative atom. As shown here, hydrogen (H) bonds can form between molecules or between different parts of the same molecule.
A A hydrogen (H) bond is an attraction between an electronegative atom and a hydrogen atom taking part in a separate polar covalent bond.
Fig. 2.9a, p. 29

15. Hydrogen Bonds in DNA
Hydrogen bonds form between different parts of the same molecule that have different electronegativities

16. Figure 2.9 Hydrogen bonds. Hydrogen bonds form at a hydrogen atom taking part in a polar covalent bond. The hydrogen atom’s slight positive charge weakly attracts an electronegative atom. As shown here, hydrogen (H) bonds can form between molecules or between different parts of the same molecule.
B Hydrogen bonds are individually weak, but many of them form. Collectively, they are strong enough to stabilize the structures of large biological molecules such as DNA, shown here.
Fig. 2.9b, p. 29

17. ANIMATION: Examples of hydrogen bonds
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18. Key Concepts Atoms Bond
Atoms of many elements interact by acquiring, sharing, and giving up electrons
Interacting atoms may form ionic, covalent, or hydrogen bonds

19. ANIMATION: Isotopes of hydrogen19

20. ANIMATION: Covalent bonds20

21. 2.5 Water’s Life-Giving Properties
Water is essential to life because of its unique properties:
Solvent for salts and other polar solutes
Resists temperature changes
Cohesion
Unique properties of water result from extensive hydrogen bonding among water molecules

22. Each Water Molecule Is Polar
Polar covalent bonds join two hydrogen atoms to one oxygen atom in each water molecule

23. slight negative chargeO H H
slight positive charge
slight positive charge
p. 30

24. Hydrogen Bonding in Water
Polarity invites extensive hydrogen bonding between water molecules
Figure 2.10 Water. Many hydrogen bonds (dashed lines) that quickly form and break keep water molecules clustered together tightly. The extensive hydrogen bonding in liquid water gives it unique properties.

25. ANIMATION: Structure of water
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26. Water Is an Excellent Solvent
Water is a solvent that easily dissolves salts, sugars, and other polar substances
solvent
Liquid that can dissolve other substances
solute
A dissolved substance

27. Salts
Sodium chloride (NaCl) is an example of a salt, which is easily dissolved in water
salt
Compound that dissolves easily in water and releases ions other than H+ and OH–

28. Dissolving a Salt
Water molecules dissolve an ionic solid such as NaCl by surrounding the atoms and pulling them apart
Figure 2.11 Water molecules that surround an ionic solid pull its atoms apart, thereby dissolving them.

29. ANIMATION: Dissolution29

30. Hydrophilic and Hydrophobic
Hydrophilic substances dissolve easily in water; hydrophobic substances do not
hydrophilic
Substance that dissolves easily in water, such as salt
hydrophobic
Substance that resists dissolving in water, such as oil

31. Cohesion
Hydrogen bonds cause water molecules to resist separating from each other
cohesion
Tendency of molecules to stick together
Pulls water upward in plants
Causes surface tension
Figure 2.12 Visible effect of cohesion: a wasp drinking, not sinking. Cohesion imparts surface tension to liquid water, which means that the surface of liquid water behaves a bit like a sheet of elastic.

32. Evaporation Cohesion of water molecules resists evaporation
Transition of a liquid to a gas
Requires energy (removes heat from liquid)

33. Water Stabilizes Temperature
Temperature stability is important for homeostasis; most molecules of life function within a certain temperature range
Because of hydrogen bonding, it takes more heat to raise the temperature of water compared with other liquids
temperature
Measure of molecular motion

34. Ice
Ice forms below about 0°C (32°F), as hydrogen bonds lock water molecules in a rigid, three-dimensional lattice
Ice floats because the molecules pack less densely than in water

35. Ice Floats on Water
Figure 2.13 Ice floats on water. Left, a covering of ice can insulate water underneath it, thus keeping aquatic organisms from freezing during harsh winters. Right, ice forms below about 0ｰC (32ｰF), as hydrogen bonds lock water molecules in a rigid, three-dimensional lattice. It floats because the molecules pack less densely than in water.

36. Key Concepts Water of Life
Water stabilizes temperature, has cohesion, and can act as a solvent for many other substances
These properties make life possible

37. ANIMATION: Spheres of hydration
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38. 2.6 Acids and Bases
Most biological processes occur within a narrow range of pH, typically around pH 7 pH
Measure of concentration of hydrogen ions (H+) in a fluid
concentration
Number of molecules or ions of a solute per unit volume of a solution

39. Acids and Bases
Acids release hydrogen ions in water; bases accept them
acid
Substance that releases hydrogen ions in water
base
Substance that accepts hydrogen ions in water

40. Water
In liquid water, water molecules spontaneously separate into hydrogen ions (H+) and hydroxyl ions (OH-)
H2O (water) ↔ H+ (hydrogen ions) + OH– (hydroxide ions)
At neutral pH (7), the amounts of H+ and OH– ions are equal

41. HCl (hydrochloric acid) ↔ H+ (hydrogen ions) + Cl– (chloride ions)
Strong Acids
Strong acids give up more H+ ions than weak acids
Hydrochloric acid (HCl) is a strong acid that, when added to water, easily separates into H+ and Cl–
HCl (hydrochloric acid) ↔
H+ (hydrogen ions) + Cl– (chloride ions)

42. A pH scale pH scale ranges from 0 (most acidic) to 14 (most basic)
One unit on the scale corresponds to a tenfold change in H+ ions
Red dots = hydrogen ions (H+) and blue dots = hydroxyl ions (OH-)

43. — 0 battery acid — 1 gastric fluid — 2 acid rain lemon juice cola
more acidic
vinegar
— 3
orange juice
tomatoes, wine
— 4
bananas
beer
bread
— 5
black coffee
urine, tea, typical rain
— 6
corn
butter
milk
— 7
pure water
blood, tears
egg white
— 8
seawater
baking soda
— 9
detergents
Figure 2.14 A pH scale. Here, red dots signify hydrogen ions (H+) and blue dots signify hydroxyl ions (OH–). Also shown are approximate pH values for some common solutions. This pH scale ranges from 0 (most acidic) to 14 (most basic). A change of one unit on the scale corresponds to a tenfold change in the amount of H+ ions.
Tums
toothpaste
— 10
hand soap
milk of magnesia
more basic
— 11
household ammonia
— 12
hair remover
bleach
— 13
oven cleaner
— 14
drain cleaner
Fig. 2.14, p. 32

44. ANIMATION: The pH scale44

45. Buffers A buffer keeps a solution within a consistent range of pH
Most cell and body fluids are buffered because most molecules of life work only within a narrow range of pH
buffer
Set of chemicals that stabilize pH of a solution by alternately donating and accepting ions that contribute to pH

46. The Bicarbonate Buffer System
Carbon dioxide gas becomes a weak acid when it dissolves in the fluid portion of human blood:
H2O + CO2 (carbon dioxide) → H2CO3 (carbonic acid)
Carbonic acid separates into hydrogen ions and bicarbonate ions, which can recombine to form carbonic acid:
H2CO3 (carbonic acid) ↔ H+ + HCO3- (bicarbonate)

47. Bicarbonate Buffer System (cont.)
Exchange of ions between carbonic acid and bicarbonate keeps blood pH between 7.3 and 7.5 – up to a point
Buffer failure can be catastrophic in a biological system
Example: Too much carbonic acid forms in blood when breathing is impaired suddenly – the resulting decline in blood pH may cause coma

48. Acid Rain
Burning fossil fuels releases acid sulfur and nitrogen compounds
Rain and fog can be more acid than vinegar
Figure 2.15 Corrosive effect of acid rain. Airborne pollutants dissolve in water vapor and form compounds that change the pH of rain.

49. Key Concepts The Power of Hydrogen
Most of the chemistry of life occurs in a narrow range of pH, so the fluids inside organisms are buffered to stay within that range

50. Mercury Rising (revisited)
Today, all ecosystems on Earth have detectable effects of air pollution
The amount of mercury in Earth’s waters is rising

51. Summary: Chemistry of Life