The Chemical Basis of Life
Energy levels of electrons
*chemical
behavior due to
electrons
Electron configuration and
chemical properties:
Covalent Bonds
– A covalent bond forms when two atoms share one or
more pairs of outer-shell electrons.
– Atoms held together by covalent bonds form a
molecule.
Name
Electron configuration
molecular formula
Hydrogen gas H2
Structural formula
Space-filling model
Single bond
a pair of shared electrons
Oxygen gas O2
Double bond
two pairs of shared electrons
Methane CH4
Ball-and-stick model
Ionic Bonds
– When an atom loses or gains electrons, it becomes
electrically charged.
• Charged atoms are called ions.
• Ionic bonds are formed between oppositely charged ions.
Outer shell
has 1 electron
The outer electron is stripped
from sodium and completes
the chlorine atom’s outer shell
Complete
outer shells
Outer shell
has 7 electrons
Na
Sodium atom
Cl
Chlorine atom
Na
Sodium ion
Cl
Chlorine ion
Sodium chloride (NaCl)
The attraction
between the
ions—an ionic
bond—holds
them together
Hydrogen Bonds
– Water is a compound in which the electrons in its
covalent bonds are shared unequally.
• This causes water to be a polar molecule, one with opposite
charges on opposite ends.
slightly 
slightly 
H
H
O
slightly –
Hydrogen bonding
• Weak bonds formed
between hydrogen and
another atom
– Surface tension of water
• Important as
intramolecular bonds,
giving shape to proteins
and other biomolecules
WATER AND LIFE
– Life on Earth began in water and evolved there for 3
billion years.
• Modern life remains tied to water.
• Your cells are composed of 70%–95% water.
Properties of Water
• Numerous properties due to the hydrogen bonding
Properties of Water
• Cohesion
• Adhesion
Evaporation from
the leaves
Microscopic tubes
SEM
Cohesion due to
hydrogen bonds
between water
molecules
Properties of Water
– Heat and temperature
– Water can absorb and store large amounts of heat
while only changing a few degrees in temperature.
– Water can moderate temperatures.
Properties of Water
– Surface tension is the measure of how difficult it is
to stretch or break the surface of a liquid.
Water is the solvent of life
*solution
*solvent
*solute
*aqueous solution
The Chemical Basis of Life
Hydrophilic and hydrophobic substances
*hydrophilic
*hydrophobic
The dissociation of water molecules
H2O
H+ + OH-
Acids and bases
*acid
*base
Acids, Bases and pH
Oven cleaner
Increasingly basic
lower H concentration
Basic
solution
14
Neutral
solution
Acidic
solution
Increasingly acidic
greater H concentration
Neutral
[H+]  [OH–]
13
Household
12 bleach
Household ammonia
11
Milk of magnesia
10
9
Seawater
8
Human blood
7 Pure water
6 Urine
5
4 Tomato juice
3 Grapefruit juice,
soft drink
2 Lemon juice,
gastric juice
1
0
pH scale
Carbon and Organic Chemistry
– Carbon is a versatile atom.
– Carbon forms large, complex, and diverse molecules
necessary for life’s functions.
– Organic compounds are carbon-based molecules.
Structural
formula
Ball-and-stick
model
Space-filling
model
Carbon and Organic Chemistry
• Variations in Carbon
skeletons
Carbon skeletons vary in length
Carbon skeletons may be unbranched or branched
Carbon skeletons may have double bonds,
which can vary in location
Carbon skeletons may be arranged in rings
Hydrocarbons
– Larger hydrocarbons form fuels for engines.
– Hydrocarbons of fat molecules fuel our bodies.
Chemical Components of Cells
*structural isomers
*geometric isomers
*enantiomers
Example of enantiomers: