Chapter 2
• Atoms, Molecules, and Life
Copyright © 2005 Pearson Prentice Hall, Inc.

2.1 What Are Atoms?
• 2.1.1 Atoms, the Basic Structural Units of Matter, Are Composed of Still
Smaller Particles
– Figure 2.1 Atomic models (p. 22)
– Table 2.1 Common Elements Important in
Living Organisms (p. 22)
– Figure E2.1 How positron emission tomography works (p.23)
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e electron shell p +
Hydrogen (H) nucleus e p + p + n n e -
Helium (He)

e
electron shell p +
Hydrogen (H) nucleus

e
p
+ p
+ n n e
-
Helium (He)

2.1 What Are Atoms?
– 2.1.1.1 Electrons Orbit the Nucleus at
Fixed Distances, Forming Electron Shells
That Correspond to Different Energy Levels
• Figure 2.2 Electron shells in atoms (p. 24)
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4 e
–
2 e
–
6 p +
6 n
6 e
–
2 e
–
8 p +
8 n
5 e
–
8 e
–
2 e
–
15 p +
16 n
Carbon (C)
C
Oxygen (O)
O
Phosphorus (P)
P
2 e
–
8 e
–
8 e
–
2 e
–
20 p +
20 n
Calcium (Ca)
Ca

2.2 How Do Atoms Interact to
Form Molecules?
• 2.2.1 Atoms Interact with Other Atoms
When There Are Vacancies in Their
Outermost Electron Shells
• 2.2.2 Charged Atoms Called Ions
Interact to Form Ionic Bonds
– Figure 2.3 (Hide/Reveal) The formation of ions and ionic bonds (p. 25)
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Na Cl
Sodium atom (neutral) Chlorine atom (neutral)
11 p +
11 n
17
18 p n
+
Na +
Sodium ion (+)
Cl
–
Chloride ion ( –)
11 p +
11 n
17
18 p n
+
Na +
Cl
–
An ionic compound: NaCl

Na
Sodium atom (neutral)
Cl
Chlorine atom (neutral)
11 p
11 n
+
17 p
18 n
+

Na +
Sodium ion (+)
Cl
–
Chloride ion ( –)
11 p
+
11 n
17
18 p n
+

Na
+
Cl
–
An ionic compound: NaCl

2.2 How Do Atoms Interact to
Form Molecules?
• 2.2.3 Uncharged Atoms Can Become
Stable by Sharing Electrons, Forming
Covalent Bonds
– Figure 2.4 Covalent bonds involve shared electrons (p. 26)
– Table 2.2 Chemical Bonds (p. 26)
Copyright © 2005 Pearson Prentice Hall, Inc.

Nonpolar covalent bonding
Hydrogen (H –H or H
2
)
8 p +
8 n
Oxygen (O=O or O
2
)
8 p +
8 n
Polar covalent bonding
(slightly negative)
8 p +
8 n
(slightly positive)
Water (H
–O–H or H
2
O)

2.2 How Do Atoms Interact to
Form Molecules?
– 2.2.3.1 Most Biological Molecules Utilize
Covalent Bonding
• Unnumbered Figure 3 Bonding Patterns of
Atoms Commonly Found in Biological
Molecules (p. 27)
• Table 2.3 Bonding Patterns of Atoms
Commonly Found in Biological Molecules (p.
27)
Copyright © 2005 Pearson Prentice Hall, Inc.

2.2 How Do Atoms Interact to
Form Molecules?
– 2.2.3.2 Electron Sharing Determines
Whether a Covalent Bond Is Nonpolar or
Polar
– 2.2.3.3 Free Radicals Are Highly Reactive and Can Damage Cells
• 2.2.4 Hydrogen Bonds Are Weaker
Electrical Attractions Between or Within
Molecules with Polar Covalent Bonds
– Figure 2.5 Hydrogen bonds (p. 28)
Copyright © 2005 Pearson Prentice Hall, Inc.

H
(+)
O
(
–)
H
(+)
H
(+)
O
( –)
H
(+) hydrogren bonds

2.3 Why Is Water So
Important to Life?
• 2.3.1 Water Interacts with Many Other
Molecules
– Figure 2.6 Water as a solvent (p. 29)
– Figure 2.7 Water dissolves many biological molecules (p. 29)
Copyright © 2005 Pearson Prentice Hall, Inc.

Na
+
Cl
–
H
H
O
–
Na
+
Cl
–
H
O
Na +
H

hydroxyl group water hydrogen bond glucose

2.3 Why Is Water So
Important to Life?
• 2.3.2 Water Molecules Tend to Stick
Together
– Figure 2.8 Cohesion among water molecules (p. 30)
– Unnumbered Figure 1 Ionization of Water
(p. 30)
Copyright © 2005 Pearson Prentice Hall, Inc.

H
O
H water
(H
2
O)
H
O
( –)
+ H
(+) hydroxide ion
(OH
–
) hydrogen ion
(H + )

2.3 Why Is Water So
Important to Life?
• 2.3.3 Water-Based Solutions Can Be
Acidic, Basic, or Neutral
– Figure 2.9 The pH scale (p. 31)
– Unnumbered Figure 2 Comparison of liquid and solid phases of water (p. 32)
Copyright © 2005 Pearson Prentice Hall, Inc.

H+ concentration
(moles/liter)
10
0
10
–1
10
–2
10
–3
10
–4
10
–5
10
10
10 pH value
0
1
2
3
4
1-molar hydrochloric acid (HCI) stomach acid lime juice lemon juice
"acid rain" (2.5
–5.5) vinegar, cola, orange juice, tomatoes beer
5 black coffee, tea
6 normal rain (5.6) urine (5.7)
(H neutral
+
= OH
–
)
7
8 pure water (7.0) saliva blood, sweat (7.4) seawater (7.8–8.3)
10
10
10
10
9 baking soda
10
11 phosphate detergents chlorine bleach milk of magnesia household ammonia some detergents
(without phosphates)
12 washing soda
10
10
13 oven cleaner
14 1-molar sodium hydroxide (NaOH)

H
+ concentration
(moles/liter)
10
0
10
–1
10
–2
10
–3
10
–1
10
–5
10
–6 pH value
0 1-molar hydrochloric acid (HCI)
1 stomach acid lime juice
2 lemon juice
3 "acid rain" (2.5
–5.5) vinegar, cola, orange juice, tomatoes
4 beer
5 black coffee, tea
6 normal rain (5.6) urine (5.7)

10
–7
10
–8
10
–9
10
–10
10
–11
10
–12
10
–13
10
–14 neutral
(H + = OH
–
)
7
8 pure water (7.0) aliva blood, sweat (7.4) seawater (7.8
–8.3)
9 baking soda
10
11 phosphate detergents chlorine bleach milk of magnesia household ammonia some detergents
(without phosphates)
12 washing soda
13 oven cleaner
14 1-molar sodium hydroxide (NaOH)

2.3 Why Is Water So
Important to Life?
– 2.3.3.1 A Buffer Helps Maintain a Solution at a Relatively Constant pH
• 2.3.4 Water Moderates the Effects of
Temperature Changes
• 2.3.5 Water Forms an Unusual Solid:
Ice
– Figure E2.2 Chocolate (p. 33)
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