1. The Chemistry of Life
Chapters 2

2. Think about it…
Why must we understand chemistry first in order to understand biology? Write down 3 reasons. Try and be specific!

3. Chapter 2: The Chemical Basis of Life

4. Elements, Atoms, and Molecules
Living organisms are composed of matter
Matter is composed of elements
Element - substance that cannot be broken down
92 elements in nature—only a few exist in a pure state
Life requires 25 essential elements; some are called trace elements

5. Elements, Atoms, and Molecules
Compound—substance of 2+ different elements combined in a fixed ratio
many compounds that consist of only 2 elements
Table salt +
Sodium
Chlorine
Sodium Chloride

6. Elements, Atoms, and Molecules
An atom is the smallest unit of matter
made of 100+ subatomic particles
Proton—positive electrical charge
Electron—negative electrical charge
Neutron—electrically neutral
Elements differ in # of protons, neutrons, and electrons
Protons and neutrons - in the nucleus
electrons orbit around the nucleus
How do electrons stay orbiting around the nucleus?

7. Electron cloud Nucleus 2e–
Figure 2.4A Two models of a helium atom 2
Protons
Mass
number = 4 2
Neutrons 2
Electrons

8. Elements, Atoms, and Molecules
The number of protons = atomic number
Electron
cloud
6e–
Nucleus 6
Protons
Mass
number = 12 6
Neutrons 6
Electrons

10. Elements, Atoms, and Molecules
all atoms of an element have the same atomic number, some differ in mass number
Isotopes = same numbers of protons and electrons; different numbers of neutrons

11. Radioactive Isotopes
Read page 21 on radioactive isotopes and do the following:
How are radioactive isotopes used in research?
How are they used in diagnoses of medical diseases?
What are the dangers of using tracers in medicine

12. Elements, Atoms, and Molecules
Only electrons are involved in chemical activity
Electrons occur in energy levels called electron shells
Hydrogen
Helium
First
shell
Lithium
Beryllium
Boron
Carbon
Nitrogen
Oxygen
Fluorine
Neon
Second
shell
Sodium
Magnesium
Aluminum
Silicon
Phosphorus
Sulfur
Chlorine
Argon
Third
shell

13. Concept Check
1. The reactive properties or chemical behavior of an atom mostly depend on the number of
a. the electrons in each electron shell of the atom.
b. the neutrons found in the nucleus.
c. the filled electron shells.
d. the electrons in the outer electron shell of the atom.
Answer: 4

14. Elements, Atoms, and Molecules
Atoms want to fill their outer electron shells
atoms share, donate, or receive electrons
This results in attractions between atoms called chemical bonds
Ionic bonds vs. covalent bonds

15. Elements, Atoms, and Molecules
ion – atom/molecule with a +/- charge
When an electron is lost, a + charge results; when one is gained, a - charge results
Two ions with opposite charges attract each other
When the attraction holds ions together, it is called an ionic bond

16. Sodium chloride (NaCl)
Transfer of
electron + – Na
Sodium atom Cl
Chlorine atom
Na+
Sodium ion
Cl–
Chloride ion
Figure 2.7A Formation of an ionic bond, producing sodium chloride.
Sodium chloride (NaCl)

17. Elements, Atoms, and Molecules
covalent bond - when atoms share outer-shell electrons
A molecule is formed when atoms are held together by covalent bonds

18. Elements, Atoms, and Molecules
Atoms in a covalently bonded molecule compete for shared electrons
attraction (pull) for shared electrons is called electronegativity
More electronegative = pull harder
In molecules of only 1 element, the pull toward each atom is =, because each atom has the same electronegativity
The bonds formed are called nonpolar covalent bonds

19. Elements, Atoms, and Molecules
Water has atoms w/ different electronegativities
Oxygen attracts the shared electrons more strongly than hydrogen
So, the shared electrons spend more time near oxygen
The result is a polar covalent bond
(–)
(–) O H H
(+)
(+)

20. Elements, Atoms, and Molecules
Some chemical bonds are weak
Water molecules are electrically attracted to oppositely charged regions on neighboring molecules
Because the positively charged region is always a hydrogen atom, the bond is called a hydrogen bond
Hydrogen bond

21. Water
Hydrogen bonding causes molecules to stick together, called cohesion
help transport water and nutrients up the plant
Cohesion is related to surface tension—a measure of how difficult it is to break the surface of a liquid

22. Adhesion Water-conducting cells Cohesion 150 µm
Direction
of water
movement
Cohesion
Campbell, Neil, and Jane Reece, Biology, 8th ed., Figure 3.3 Water Transport in Plants.
150 µm

23. What will break hydrogen bonds?
Water
Adhesion – when water molecules stick to other substances.
What will break hydrogen bonds?

24. Water
solution - a liquid consisting of a uniform mixture of two or more substances
dissolving agent = solvent
substance dissolved = solute
Suspension - a substance that doesn’t dissolve but separates into tiny pieces
Water keeps pieces suspended so they don’t settle out

25. Ion in Salt solution crystal
Figure 2.14 A crystal of salt (NaCl) dissolving in water.
Ion in
solution
Salt
crystal

26. Water water molecules can break apart into ions
Some are hydrogen ions (H+)
Some are hydroxide ions (OH–)
Both are reactive
Chemicals other than water can contribute H+ to a solution = acids
An acidic solution has a higher concentration of H+ than OH–

27. Water
Some chemicals accept hydrogen ions and remove them from solution = bases
For example, sodium hydroxide (NaOH) provides OH– that combines with H+ to produce H2O (water)
A pH scale - used to describe whether a solution is acidic or basic
pH ranges from 0 (most acidic) to 14 (most basic)
A solution that is neither acidic or basic is neutral (pH = 7)

28. (Higher concentration of H+) (Lower concentration of H+)
pH scale 1
Battery acid 2
Lemon juice, gastric juice 3
Grapefruit juice, soft drink,
vinegar, beer
(Higher concentration of H+)
Increasingly ACIDIC
Acidic solution 4
Tomato juice 5
Rain water 6
Human urine
Saliva
NEUTRAL
[H+]=OH–] 7
Pure water
Human blood,
tears 8
Seawater
Neutral solution 9
Figure 2.15 The pH scale represents the relative concentration of H+ and OH–. 10
(Lower concentration of H+)
Increasingly BASIC
Milk of magnesia 11
Household ammonia 12
Household bleach 13
Oven cleaner
Basic solution 14

29. Acidic solution Basic solution Neutral solution
Figure 2.15 The pH scale represents the relative concentration of H+ and OH–.
Acidic solution
Neutral solution
Basic solution

30. Reactions
The formation of water from hydrogen and oxygen is an example of a chemical reaction
The reactants (H2 and O) are converted to H2O, the product
What are a few examples of reactions that living things (plants/animals) must go through in order to survive? Find the reaction equation for them. What are the products/reactants?

31. Interpreting Data
3. This is the general equation for photosynthesis—the process of capturing sunlight energy and converting it to chemical energy. Which of the following are the reactants of this reaction?
a. C6H12O6 and O2.
b. CO2 and H2O.
Answer: 2