1. Atoms, Molecules, and Life
Chapter 2
Atoms, Molecules, and Life

2. 2.1 What Are Atoms?
Elements: substances that can neither be ______________ nor converted to other substances (e.g., carbon)

3. Atomic number: the number of __________ in the nucleus
2.1 What Are Atoms?
Atoms: basic structural unit of _________; made up of subatomic particles
Atomic nucleus (central part of the atom)
___________ (positive charge)
Neutrons (neutral charge)
Electrons (negative charge)
Atomic number: the number of __________ in the nucleus
Is unique for each element
92 different elements have been described.

4. Hydrogen and helium have the _____________ atomic structure.
2.1 What Are Atoms?
Hydrogen and helium have the _____________ atomic structure.
Hydrogen has one proton and one electron.
Helium has two protons, two neutrons, and two electrons. e- e- p+ p+ p+ n n e-
atomic nucleus
(a) Hydrogen (H)
(b) Helium (He)
Fig. 2-1

5. 2.1 What Are Atoms?
Atoms of the same _____________ with different numbers of _______________ are called isotopes of the element.
Some isotopes spontaneously break apart, forming different kinds of atoms and releasing ______________ in the process.
Such isotopes are _______________.
Example: radioactive uranium isotopes decay and form lead in the process

6. 2.1 What Are Atoms?
PLAY
Animation—Atomic Structure

7. Different atoms have different electron shells:
2.1 What Are Atoms?
Electron shells: electrons orbit around _____________ at specific distances, called electron shells.
Different atoms have different electron shells:
The inner shell only has ________ electrons.
The second shell holds up to _________ electrons.
Additional shells hold up to ____________ electrons.

8. The first four atomic electron shells
2.1 What Are Atoms?
The first four atomic electron shells
Carbon (C)
Oxygen (O)
Phosphorus (P)
Calcium (Ca)
2e-
5e-
8e-
4e-
6e-
8e-
8e-
2e-
2e-
2e-
2e-
6p+
8p+
15p+
20p+ 6n 8n
16n
20n
Carbon (C)
Oxygen (O)
Phosphorus (P)
Calcium (Ca) C O P Ca
Fig. 2-2

9. Electrons can move from ___________to electron shell.
2.1 What Are Atoms?
Electrons can move from ___________to electron shell.
Electrons move from an _______ to an outer shell when absorbing energy.
Electrons move from an ______ shell to an inner shell when releasing energy. 2
The energy boosts the electron to a higher-energy shell 1
An electron absorbs energy
energy – – + + 3
The electron drops back into lower-energy shell, releasing energy as light
light – +
Fig. 2-3

10. 2.2 How Do Atoms Form Molecules?
Molecules: _______or more atoms of one or more elements held together by interactions among their _____________electron shells
Atoms interact with one another according to two basic principles:
An inert atom will not react with other atoms when its outermost electron shell is completely ______ or ___________.
A reactive atom will react with other atoms when its outermost electron shell is only ______________.

11. 2.2 How Do Atoms Form Molecules?
Atoms combine with each other to fill outer electron shells (e.g. hydrogen and oxygen have unfilled outer electron shells, and thus, can combine to form the water molecule).
The water molecule, with a filled outer electron shell, is more _________ than either the hydrogen or oxygen atoms that gave rise to it.
The results of losing, gaining, or sharing electrons are chemical bonds—attractive forces that hold atoms together in molecules.

12. 2.2 How Do Atoms Form Molecules?
PLAY
Animation—Biologically Important Atoms

13. 2.2 How Do Atoms Form Molecules?
A molecule may be depicted in different ways. H C C C C O H H H H H
(a) All bonds shown
CH3
CH2
CH2
CH2 OH
(b) Bonds within common groups omitted OH
(c) Carbons and their attached hydrogens omitted
(d) Overall shape depicted
Fig. 2-4

14. 2.2 How Do Atoms Form Molecules?
Types of bonds
__________ bonds: formed by passing an electron from one atom to another
One partner becomes positive, the other negative, and they ________ one another.
Na+ + Cl– becomes NaCl (sodium chloride)
Positively or negatively charged atoms are called _____________.

15. 2.2 How Do Atoms Form Molecules?
Sodium atom (neutral)
Chlorine atom (neutral) – – –
Charged atoms interact to form ionic bonds.
Positively charged atoms
Negatively charged atoms – – – – – – – – – – – – – – –
11p+
17p+ –
11n – –
18n – – – – – – –
Electron transferred
(a)
Neutral atoms
Sodium ion (+)
Chloride ion (–) – – – – – – – – – – – – – – – – – –
11p+
17p+ –
11n – –
18n – – – – – – –
Attraction between opposite charges
(b)
Ions
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Fig. 2-5
(c)
An ionic compound: NaCl

16. 2.2 How Do Atoms Form Molecules?
PLAY
Animation—Ionic Bonds

17. 2.2 How Do Atoms Form Molecules?
Types of bonds (continued)
Covalent bonds: bond between two atoms that _________ electrons in their outer electron shell
For example, an H atom can become stable by sharing its electron with another H atom, forming H2 gas.

18. 2.2 How Do Atoms Form Molecules?
Covalent bonds produce either _______ or ___________ molecules.
Nonpolar molecule: atoms in a molecule ______________ share electrons that spend equal time around each atom, producing a nonpolar covalent bond

19. 2.2 How Do Atoms Form Molecules?
Nonpolar covalent bonding in hydrogen
Same charge on both nuclei – + + –
Electrons spend equal time near each nucleus
(uncharged)
(a)
Nonpolar covalent bonding in hydrogen
Fig. 2-6a

20. 2.2 How Do Atoms Form Molecules?
Covalent bonds produce either nonpolar or polar molecules (continued).
Polar molecules: atoms in a bond _____________ share electrons, producing a polar covalent bond
One atom in the bond has a more positive charge in the___________, and so attracts electrons more strongly, becoming the negative pole of the molecule.
The atom in the bond that has a less positive charge in the nucleus gives up_________, becoming the ______pole of the molecule.

21. 2.2 How Do Atoms Form Molecules?
Polar covalent bonding in water
(oxygen: slightly negative)
(–)
Larger positive charge – – – – –
Electrons spend more time near the larger nucleus – – –
8p+ – 8n – + +
Smaller positive charge
(hydrogens: slightly positive)
(+)
(+)
(b)
Polar covalent bonding in water
Fig. 2-6b

22. 2.2 How Do Atoms Form Molecules?
PLAY
Animation—Covalent Bonds

23. 2.2 How Do Atoms Form Molecules?
Types of bonds (continued)
Hydrogen bonds: __________ electrical attraction between positive and negative parts of polar molecules
Example: the negative charge of oxygen atoms in water molecules attract the positive charge of hydrogen atoms in other water molecules

24. 2.2 How Do Atoms Form Molecules?
Hydrogen bonds
O (–)
H (+)
H (+)
H (+)
O (–)
H (+)
hydrogen bonds
Fig. 2-7

25. 2.2 How Do Atoms Form Molecules?
PLAY
Animation—Introducing Water’s Properties

26. 2.2 How Do Atoms Form Molecules?

27. 2.3 Why Is Water So Important To Life?
Water interacts with many other molecules.
Oxygen released by plants during photosynthesis comes from water.
Water is used by animals to digest food.
Water is produced in chemical reactions that produce_______ , fats, and sugars.

28. 2.3 Why Is Water So Important To Life?
Many molecules dissolve easily in water.
Water is an excellent________ , capable of dissolving a wide range of substances because of its positive and negative poles.
NaCl dropped into H2O
The positive end of H2O is attracted to Cl–.
The negative end of H2O is attracted to Na+.
These attractions tend to push apart the components of the original salt.

29. 2.3 Why Is Water So Important To Life?
Water as a solvent
Cl–
Na+ H
Na+
Cl– O H
Cl–
Na+
Fig. 2-8

30. 2.3 Why Is Water So Important To Life?
PLAY
Animation—Solvent

31. 2.3 Why Is Water So Important To Life?
Water molecules tend to stick together.
Surface tension: water tends to resist being__________
________________ : water molecules stick together
Fig. 2-9

32. 2.3 Why Is Water So Important To Life?
PLAY
Animation—High Cohesion

33. 2.3 Why Is Water So Important To Life?
Water can form ions.
Water spontaneously becomes H+ and OH–.
Acid solutions have a lot of H+ (__________).
Alkaline solutions have a lot of OH– (hydroxyl ions).
A base is a substance that combines with H+, reducing their numbers.
pH measures the relative amount of _________ and OH– in a solution.

34. 2.3 Why Is Water So Important To Life?
A water molecule is ionized.
(–)
(+) O O + H H H H
water
(H2O)
hydroxide ion (OH–)
hydrogen ion (H+)
Fig. 2-10

35. 2.3 Why Is Water So Important To Life?
pH measures acidity.
Acids have a pH below 7.
Bases have a pH above 7.
Neutral solutions have a pH of 7.
Buffers are substances that ___________ a constant pH in a solution.

36. 2.3 Why Is Water So Important To Life?
The pH scale
stomach acid (2) lemon juice (2.3)
vinegar, cola (3.0)
1 molar hydrochloric acid (HCl)
black coffee (5.0)
normal rain (5.6) urine (5.7)
orange (3.5) tomatoes beer (4.1)
water from faucet milk (6.4) pure water (7.0)
household ammonia (11.9) washing soda (12)
drain cleaner (14.0) 1 molar sodium hydroxide (NaOH)
blood, sweat (7.4)
phosphate detergents chlorine bleach (12.6)
seawater (7.8–8.3)
baking soda (8.4)
oven cleaner (13.0)
toothpaste (9.9) 1 2 3 4
pH value 5 6 7 8 9 10 11 12 13 14
(H+ > OH–)
(H+ < OH–)
neutral (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
increasingly acidic
increasingly basic
H+ concentration in moles/liter
Fig. 2-11

37. 2.3 Why Is Water So Important To Life?
PLAY
Animation—pH Scale