Chapter 2:
The Chemistry of Life
PowerPoint modified from one by Mrs. Fisch
Atom - from Greek “atomos” = unable to be cut
Atomic Structure
• 2 regions:
– Nucleus: the center of the atom,
contains nearly all of the mass of the
atom
– Electron cloud: region that surrounds
the nucleus that contains most of the
space in the atom
Nucleus
Electron
Cloud
ATOMIC STRUCTURE
All atoms are made up of subatomic particles
called protons, neutrons and electrons.
Particle
Charge
Mass
Location
Proton
Positive
charge
1
Inside
nucleus
Neutron
No
1
Inside
charge
nucleus
Electron Negative Close to Orbiting
charge
0
nucleus
Where are these particles located?
Nucleus
Electron
Cloud
Proton
proton
+
Electron
N
N
+
neutron
Neutron
Where are these particles located?
Nucleus
Electron
Cloud
Proton
proton
+
Electron
N
N
+
neutron
Neutron
• Atomic number= number of
protons in an atom of an element.
• All atoms of an element have the
same atomic number.
• Example: Every hydrogen atom has
one proton in its nucleus. Hydrogen
is assigned the atomic number 1.
How many protons?
A The atomic number of sulfur (S) is 16.
Sulfur has 16 protons in its nucleus
B The atomic number of iron (Fe) is 26.
Iron has 26 protons in its nucleus
C The atomic number of silver (Ag) is 47.
Silver has 47 protons in its nucleus
Atoms are neutral, so each positive charge
in an atom is balanced by a negative
charge.
So the atomic number equals the
number of electrons.
Mass Number
The mass number= the sum of the protons
+ neutrons of that atom.
Question: Aluminum’s atomic number is 13.
It’s mass number is 27. How many neutrons
does Aluminum have?
Answer: 14
ATOMIC STRUCTURE
4
2
Mass number
He
Atomic number
the number of protons in an atom
How many protons?
How many neutrons?
How many electrons?
number of electrons = number of protons
EXAMPLE
133
55
Cs
1. What is the atomic number? 55
Atomic number = # of protons and # of electrons
2. How many protons? 55
3. How many electrons? 55
4. What is the mass number? 133
Mass number = sum of protons and neutrons
5. How many neutrons? 78 (133 – 55 = 78)
element – pure substance that consists
of only 1 type of atom.
Isotope – Atoms of an element that
have the same number of protons
but different numbers of neutrons
186
75
Re
Rhenium
Protons: 75
Electrons:75
Neutrons: 111
187
75
Re
Protons: 75
Electrons:75
Neutrons: 112
Rhenium Isotope
Some isotopes are radioactive, meaning
that their nuclei are unstable and break
down at a constant rate over time
Radioactive isotopes can be used:
• to determine the ages of rocks and
fossils.
• to treat cancer.
• to kill bacteria that cause food to spoil.
• as labels or “tracers” to follow the
movement of substances within an
organism.
chemical compound - a substance
formed by the chemical combination of
two or more elements in definite
proportions.
• A pure substance always has the same
composition.
• 2 Types:
– Elements - cannot be
broken down
chemically into simpler
substances
– Compounds - can be
chemically broken
down into elements
Water is a compound. All the
components are the same—H2O molecules.
• Mixture: a blend of two or more kinds of
matter, each of which retains its own identity
and properties.
• Homogeneous (solutions): Uniform in
composition (Saltwater, air, milk, alloys)
• Heterogeneous: not uniform throughout
(can be separated) (pizza, concrete, salad)
Chemical Bonds
The atoms in compounds are held
together by chemical bonds.
The electrons that are available to form
bonds are called valence electrons.
The main types of chemical bonds
are:
• ionic bonds
• covalent bonds
Ionic Bonds
An ionic bond is formed when one or more
electrons are transferred from one atom to
another.
These positively and negatively charged
atoms are known as ions.
Covalent Bonds
Sometimes electrons are shared by
atoms instead of being transferred.
Copyright Pearson Prentice Hall
A covalent bond forms when electrons are
shared between atoms.
• single covalent bond - one pair of
electrons is shared
• double bond - two pairs are shared
• triple bond - three pairs are shared
The structure that results when atoms
are joined together by covalent bonds
is called a molecule.
Some weaker types of bonds
1. Hydrogen bonds. Attraction between a hydrogen
(slightly positive) and the electrons on another atom
(usually oxygen or nitrogen).
Examples: water (section 2); DNA and RNA (section
3)
wikipedia
www.education.mrsec.wisc.edu
2. Van der Waals Forces
When molecules are
close together, a slight
attraction can develop
between the
oppositely charged
regions of nearby
molecules.
Ex. - molecules in a gecko’s
foot attract molecules in
a wall
THE PROPERTIES OF WATER :
Water has some unique properties that make this molecule
unlike ANY other. And we are made up of about 70%
29
water!
SECTION 2. PROPERTIES OF WATER
Your body is made up of mostly of water.
Approximately 85% of your brain, 80% of
your blood and 70% of your muscle is
water. Every cell in your body needs
water to live.
30
• Water is a polar molecule (slightly charged)
– Electrons are unevenly shared in the
covalent bonds (polar covalent bonds).
Oxygen is slightly negative, hydrogen
slightly positive.
– Allows it to attract charges on other
_
molecules
O
H
+
H
+
• Hydrogen bonds between the hydrogen in
one molecule and the oxygen of another
– Responsible for 3 special properties:
1. Cohesion – an attraction between
molecules of the same substance
-causes water’s tendency to stick
together
http://www.youtube.com/watch?v=s63JXdsL5L
U&feature=endscreen&NR=1
Cohesion …
Helps insects walk across
water
Surface Tension – water molecules on
the surface do not have other molecules
on all sides so they stick to the ones
closest. Helps keep the surface of the
water intact.
WATER: HIGH SURFACE TENSION
Pressure applied to water surface
Hydrogen
bond
“V”-shaped water molecules are held
together by hydrogen bonds. The
bonds are just strong enough to give
water a surface tension with net-like
properties.
36
2. Adhesion – attraction between
water molecules and different
substances
- capillary action causes water
to rise in a narrow tube against
the force of gravity
- allows water to rise
upward in plants
37
WATER: STRONG COHESIVENESS
300 ft.
6-ft.-tall
man
Water
molecule
released
into the
atmosphere
Water
molecules
pulled
upward
Water
molecule
pulled into
root system
Because of the
cohesive properties of
water, trees such as the
giant sequoia are able
to transport water
molecules from the
soil to their leaves 300
ft. above.
As each water molecule
evaporates, it pulls
additional water up
through the tree
because of the “stickyness” of the hydrogen
bonds that link the
water molecules.
38
3. Water has a high ability to hold or
give off heat (specific heat). A large
amount of energy is needed to change
the temperature.
Good for regulating temperature
(homeostasis)
Water has a high heat of vaporization. It
takes a lot of energy to change from liquid
to gas.
This is why sweating can cool you down
(heat removed from body to evaporate the
sweat)
Water has a high heat of fusion. A lot of
energy must be removed to freeze it. This
makes it difficult to freeze an organism
living in cold water.
Water is a powerful solvent, capable of
dissolving a variety of substances
Solute – substance dissolved in a solvent to
form a solution
Solvent – fluid that dissolves solutes
Example: Ice Tea – water is the solvent
and tea and sugar the solutes
Water can dissolve ions and polar
molecules. The slightly negative
oxygen atoms are attracted to
positive ions and atoms, and the
slightly positive hydrogen atoms
are attracted to negative ions and
atoms.
When a crystal of table salt is placed in warm water,
sodium and chloride ions are attracted to the polar
water molecules.
Cl -
Cl Na+
Na+
Water
Water
Density – water is unusual because it is less
dense as a solid (ice) than as a liquid.
Two effects of this:
1. Lakes and streams freeze from the top.
The ice is less dense so it stays at the top
and insulates the water below. This allows
organisms to survive in the water.
2. If a cell freezes, the water will expand,
causing the cell to burst. Ex. - vegetables
that have frozen and thawed are less
crunchy.
Other unique properties of water
Only substance that exists free in nature in all
three states (solid, liquid, and gas)
High boiling point
Low freezing point
Viscosity increases as its temperature is
lowered
A relatively high density to support animals
with no or delicate skeletal systems
Acids, Bases and pH
A water molecule can react to form
ions (positively or negatively charged atoms)
H2O 
H+
Hydrogen Ion
Acid
-
+ OH
Hydroxide Ion
Base
pH scale expresses hydrogen ion (H+) concentration
in a solution.
– ranges from 0 to 14
• neutral = 7 (H+ = OH -)
• below 7 = acid (more H+ than OH -)
• Above 7 = base (more OH- than H+)
– Each step represents a factor of 10
(ex. pH 4 has 10 times higher concentration of
H+ than pH 5)
Acid – compound that forms H+ ions in
solution.
–Sour taste
–Strong acids pH 1-3
–ex. HCl hydrochloric acid or stomach
acid
• Base – compound that produces OHions in solution.
–Also called alkaline
–Bitter taste
–Strong bases have pH 11-14
–Ex. NaOH = sodium hydroxide
• An acid releases a hydrogen ion when it dissolves
in water.
– high H+ concentration
– pH less than 7
stomach acid pH between 1 and 3
more acidic
7
• A base removes hydrogen ions from a solution.
– low H+ concentration
– pH greater than 7
bile pH between 8 and 9
more basic
8
• A neutral solution has a pH of 7.
pure water pH 7
9
Bases
Acids
pH
Buffers – weak acids or bases that react with
strong acids or bases to prevent sharp,
sudden changes in pH.
–Offer protection from extreme pH levels
–Produced naturally by organisms:
• Organisms can’t tolerate much pH
change
• Cells function best within a narrow pH
range
• Ex. Saliva and blood
Questions
1. As you go up the pH scale (toward 14), are there
more or less H+ ions?
2. How many times more acidic is pH 5 compared to
pH 7?
3. Would pH 8 be a strong acid, strong base, weak
acid or weak base?