1. Life’s Chemical Basis
Chapter 2

2. Characteristics of Atoms
Atoms are the building blocks of all substances
Made up of electrons, protons and neutrons
Electrons (e-) have a negative charge
Move around the nucleus
Charge is an electrical property
Attracts or repels other subatomic particles

3. Characteristics of Atoms
The nucleus contains protons and neutrons
Protons (p+) have a positive charge
Neutrons have no charge
Atoms differ in number of subatomic particles
Atomic number (# of protons) determines the element
Elements consist only of atoms with the same atomic number

4. Characteristics of Atoms
Isotopes
Different forms of the same element, with different numbers of neutrons
Mass number
Total protons and neutrons in a nucleus
Used to identify isotopes

5. Atoms

6. The Periodic Table Periodic table of the elements
An arrangement of the elements based on their atomic number and chemical properties
Created by Dmitry Mendeleev
Handout

7. 2.2 Putting Radioisotopes to Use
Some radioactive isotopes – radioisotopes – are used in research and medical applications

8. Radioisotopes
Henri Becquerel discovered radioisotopes of uranium in the late 1800s
Radioactive decay
Radioisotopes emit subatomic particles when their nucleus breaks down, transforming one element into another at a constant rate
Example: 14C → 14N
C-14 decays when a neutron splits into a proton and an electron
Its nucleus emits the electron, so C-14 becomes N-14

9. Tracers Tracer Examples:
Any molecule with a detectable substance attached
Examples:
CO2 tagged with 14C used to track carbon through photosynthesis
Radioactive tracers used in medical PET scans

10. A A patient is injected with a radioactive tracer and moved into a scanner like this one. Detectors that intercept radioactive decay of the tracer surround the body part of interest.
Figure 2.4
PET scanning.
Fig. 2-4a, p. 23

11. B Radioactive decay detected by the scanner
is converted into digital images
of the body’s interior. Two tumors (blue)
in and near the bowel of a cancer patient are visible in this PET scan.
tumors
Figure 2.4
PET scanning.
Fig. 2-4b, p. 23

12. 2.3 Why Electrons Matter Atoms accept, share, and donate electrons
Whether an atom will interact with other atoms depends on how many electrons it has

13. Atoms and Energy Levels
Electrons move around nuclei in orbitals
Apartment example
Each orbital holds two electrons
Each orbital corresponds to an energy level
An electron can move in only if there is a vacancy
vacancy no
vacancy

14. Why Atoms Interact
The shell model of electron orbitals diagrams electron vacancies; filled from inside out
First shell: one orbital (2 electrons)
Second shell: four orbitals (8 electrons)
Third shell: four orbitals (8 electrons)
Atoms with vacancies in their outer shell tend to donate, accept, or share electrons

15. argon 18p+, 18e− sodium 11p+, 11e– chlorine 17p+, 17e– carbon 6p+, 6e–
C) Third shell. This shell corresponds to the third energy level. It has four orbitals with room for eight electrons. Sodium has one electron in the third shell; chlorine has seven. Both have vacancies, so both form chemical bonds. Argon, with no vacancies, does not.
argon
18p+, 18e−
sodium 11p+, 11e–
chlorine 17p+, 17e–
B) Second shell. This shell, which corresponds to the second energy level, has four orbitals—room for a total of eight electrons. Carbon has six electrons: two in the first shell and four in the second. It has four vacancies. Oxygen has two vacancies. Both carbon and oxygen form chemical bonds. Neon, with no vacancies, does not.
carbon 6p+, 6e–
oxygen 8p+, 8e–
neon 10p+, 10e–
Figure 2.5
Shell models, which help us check for vacancies in atoms. Each circle, or shell, represents all orbitals at one energy level. Atoms with vacancies in the outermost shell tend to form bonds. Remember, atoms do not look anything like these flat diagrams.
A) First shell. A single shell corresponds to the first energy level, which has a single orbital that can hold two electrons. Hydrogen has only one electron in this shell, so it has one vacancy. A helium atom has two electrons (no vacancies), so it does not form bonds.
hydrogen 1p+, 1e–
helium 2p+, 2e–
Stepped Art
Fig. 2-5a, p. 24

16. Atoms and Ions Ion Electronegativity
An atom with a positive or negative charge due to loss or gain of electrons in its outer shell
Examples: Na+, Cl-
Electronegativity
A measure of an atom’s ability to pull electrons from another atom

17. Sodium ion 11p+ 11e– ______ no net charge Chlorine atom 17p+ 17e–
electron
loss
Sodium
atom
11p+
10e–
______
net
positive
charge
A) A sodium atom becomes a positively charged sodium ion (Na+) when it loses the electron in its third shell. The atom’s full second shell is now the outermost, and the atom has no vacancies.
electron
gain
Chloride
ion
17p+
18e–
______
net
negative
charge
B) A chlorine atom becomes a negatively charged chloride ion (Cl–) when it gains an electron and fills the vacancy in its third, outermost shell.
Figure 2.6
Ion formation.
Stepped Art
Fig. 2-6, p. 25

18. From Atoms to Molecules
Chemical bond
An attractive force existing between two atoms when their electrons interact
Molecule
Two or more atoms joined in chemical bonds

19. Combining Substances Compounds Mixture
Molecules consisting of two or more elements whose proportions do not vary
Example: Water (H2O)
Mixture
Two or more substances that intermingle but do not bond; proportions of each can vary

20. 2.4 What Happens When Atoms Interact?
The characteristics of a bond arise from the properties of the atoms that participate in it
The three most common types of bonds are ionic, covalent, and hydrogen bonds

21. Different Ways to Represent the Same Molecule

22. Ionic Bonding Ionic bond
A strong mutual attraction between two oppositely charged ions with a large difference in electronegativity
Example: NaCl (table salt)

23. Figure 2.7
Ionic bonds.
Fig. 2-7a, p. 26

24. Figure 2.7
Ionic bonds.
Fig. 2-7b, p. 26

25. Covalent Bonding Covalent bond
Two atoms with similar electronegativity and unpaired electrons sharing a pair of electrons
Can be stronger than ionic bonds
Atoms can share 1, 2, or 3 pairs of electrons (single, double, or triple covalent bonds)

26. Characteristics of Covalent Bonds
Nonpolar covalent bond
Atoms sharing electrons equally; formed between atoms with identical electronegativity
Polar covalent bond
Atoms with different electronegativity do not share electrons equally; one atom has a more negative charge, the other is more positive

27. Polarity
Polarity
Separation of charge into distinct positive and negative regions in a polar covalent molecule
Example: Water (H2O)

28. Molecular oxygen (O=O) Two oxygen atoms, each
Molecular hydrogen (H—H) Two hydrogen atoms, each with one proton, share two electrons in a nonpolar covalent bond.
Molecular oxygen (O=O)
Two oxygen atoms, each
with eight protons, share
four electrons in a double covalent bond.
Water molecule (H—O—H)
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.
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.
Fig. 2-8, p. 27

29. Hydrogen Bonding Hydrogen bond
A weak attraction between a highly electronegative atom and a hydrogen atom taking part in a separate polar covalent bond
Hydrogen bonds do not form molecules and are not chemical bonds
Hydrogen bonds stabilize the structures of large biological molecules

30. 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.
Fig. 2-9a, p. 27

31. 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.
Fig. 2-9b, p. 27

32. 2.5 Water’s Life-Giving Properties
Organisms are mostly water; all the chemical reactions of life are carried out in water
Water is essential to life because of its unique properties
The properties of water are a result of extensive hydrogen bonding among water molecules

33. Polarity of the Water Molecule
Overall, water (H2O) has no charge
The water molecule is polar
Oxygen atom is slightly negative
Hydrogen atoms are slightly positive
Hydrogen bonds form between water molecules
Gives water unique properties

34. Figure 2.10
Water, a substance that is essential for life.
Fig. 2-10a, p. 28

35. Figure 2.10
Water, a substance that is essential for life.
Fig. 2-10b, p. 28

36. Figure 2.10
Water, a substance that is essential for life.
Fig. 2-10c, p. 28

37. Water’s Solvent Properties
A substance (usually liquid) that can dissolve other substances (solutes)
Water is a solvent
The collective strength of many hydrogen bonds pulls ions apart and keeps them dissolved

38. Water’s Solvent Properties
Water dissolves polar molecules
Hydrogen bonds form between water molecules and other polar molecules
Polar molecules dissolved by water are hydrophilic (water-loving)
Nonpolar (hydrophobic) molecules are not dissolved by water

39. Water’s Temperature-Stabilizing Effects
Compared with other molecules, water absorbs more heat before it becomes measurably hotter
Temperature
A way to measure the energy of molecular motion
Molecules move faster as they absorb heat

40. Water’s Temperature-Stabilizing Effects
The surface temperature of water decreases during evaporation
Evaporation
Conversion of a liquid to a gas by heat energy
Ice is less dense than liquid water
Hydrogen bonds form a lattice during freezing

41. Water’s Cohesion Hydrogen bonds give water cohesion Cohesion
Provides surface tension
Draws water up from roots of plants
Cohesion
Molecules resist separation from one another

42. 2.6 Acids and Bases
Hydrogen ions have far-reaching effects because they are chemically active, and because there are so many of them
Chemical reactions involving acids and bases are important to homeostasis

43. Biological Reactions Occur In Water
Molecules in water (H2O) can separate into hydrogen ions (H+) and hydroxide ions (OH-)
H20 ↔ H+ + OH-

44. The pH Scale
pH is a measure of the number of hydrogen ions in a solution
The more hydrogen ions, the lower the pH
pH 7 is neutral (pure water)
Most life chemistry occurs around pH 7

45. 0 —
100
battery acid 1—
10–1
gastric fluid
2 —
acid rain
10–2
lemon juice
cola
more acidic
3 —
10–3
vinegar
orange juice
tomatoes, wine
4 —
10–4
bananas
beer
bread
5 —
10–5
coffee
urine, tea, typical rain
6 —
10–6
corn
butter
milk
7 —
10–7
pure water
blood, tears
8 —
egg white
10–8
seawater
baking soda
9 —
10–9
phosphate detergents
Tums
Figure 2.13
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 (blue numbers).
toothpaste
10 —
10–10
hand soap
milk of magnesia
more basic
11—
10–11
household ammonia
12 —
10–12
hair remover
bleach
13 —
10–13
oven cleaner
14 —
10–14
drain cleaner
Fig. 2-13, p. 30

46. How Do Acids and Bases Differ?
Acids donate hydrogen ions in a water solution
pH below 7
Bases accept hydrogen ions in a water solution
pH above 7

47. Acids: Weak or Strong Acids and bases can be weak or strong
Gastric fluid, pH 2-3
Acid rain
Example: Hydrochloric acid is a strong acid
HCl ↔ H+ + Cl-

48. Acid Rain
Sulfur dioxide dissolves in water vapor to form an acidic solution

49. Salts and Water Salt HCl (acid) + NaOH (base) → NaCl (salt) + H20
A compound that dissolves easily in water and releases ions other than H+ and OH-
HCl (acid) + NaOH (base) → NaCl (salt) + H20

50. Buffers Against Shifts in pH
Buffer system
A set of chemicals (a weak acid or base and its salt) that can keep the pH of a solution stable
OH- + H2CO3 (carbonic acid) →
HCO3- (bicarbonate) + H20
H+ + HCO3- (bicarbonate) →
H2CO3 (carbonic acid)

51. Buffering Carbon Dioxide in Blood
Carbon dioxide in blood forms carbonic acid, which separates into H+ and bicarbonate
H2O + CO2 (carbon dioxide) →
H2CO3 (carbonic acid) →
H+ + HCO3- (bicarbonate)

52. Summary: Players in the Chemistry of Life