CHAPTER 2
ATOMS, MOLECULES, AND
CHEMICAL BONDS
Matter
•Chemistry is the study of matter
•Matter takes up space and has
mass
•Matter comes in many different
forms
Chemical Elements
• Cannot be broken down
• 92 elements occurring in nature
• Symbol (different languages) 2 or 3
letters
• 25 elements are essential to life
•C, O, H, N
•Ca, P, K, S
•Fe, I
These are the
most common.
Compounds
Sulfuric Acid molecule
• Two or more elements
combined in a fixed ratio
• More common than pure
elements
WHY?
• Example: Na reacts?
Cl reacts?
NaCl reacts?
Isotopes
- different atomic forms of
the same element just
different # of neutrons
Radioactive isotopes –
unstable because the
nucleus decays
spontaneously by
giving off particles and
energy
Helpful: biological tracers (plant
studies, medicine, PET: positronemission tomography)
Harmful: damages cellular
molecules (DNA)
Half-life – time it takes for 50% of
the radioactive atoms to decay
Energy Levels
• Why does an atom behave like it
does?
• How much energy will an atom
possess?
• Electron shells – different states of
potential energy due to position in
relation to nucleus (closer to nucleus,
lower energy)
Orbital – 3-D shape where an electron
is found
How many in the 1st electron shell?
How many in the outermost shell?
So the chemical properties of an atom
depend on the number of electrons in
the outermost shell.
Hydrogen Bonds
-a hydrogen atom covalently bonds
to one electronegative atom and is
attracted to another
electronegative atom
H2O
Ex: water to water
H2O
Cohesiveness
Cohesion – water molecules stick
together because of hydrogen
bonding
Surface tension – difficult to
stretch or break the surface of
water (beads up)
Adhesion – clinging of one type of
substance to another type
Hydrophilic – water loving (clings to
water)
Capillary action – rising up a tube
made of hydrophilic material
(adhesion and cohesion together)
Imbibition – water soaks into a
porous material
Moderating Temperature
Specific heat – amount of energy it
takes to raise 1 Kg of a substance
1o C
--hydrogen bonds allow water to
resist quick temperature changes
Heat – amount of energy associated
with the movement of atoms and
molecules
Temperature – measures intensity of
heat (average speed of atoms and
molecules)
•Heated water breaks hydrogen bonds
--Molecules move faster and water
absorbs a lot of heat (but temp. only
warms a few degrees)
•Cooled water reforms hydrogen bonds
--Molecules move slower and water
releases a lot of heat (but only cools a
few degrees)
•Water requires a lot of heat to
completely break all hydrogen bonds of
a single water molecule so this
decreases water’s tendency to evaporate
--This is what gives water a high boiling
point
--When water does evaporate (heat
energy is carried off), the remaining
liquid water is cooled. Ex: sweating
Freezing of Water
• Why is water less dense as a
solid?
• What is the significance of 4
degrees Celsius in water?
--So water freezes when its
molecules no longer move
enough to break the hydrogen
bonds
Acids, Bases, and pH
When water molecules bond, the
H-bond shifts around from one
molecule to another allowing for
a Hydrogen Ion and a
Hydroxide Ion
Dissociation – separation of a water
molecule
• An acid adds [H+] to a solution
• A base reduces [H+] in a solution
so now it has more [OH-]
The product is the pH scale
(potential hydrogen), which
compresses the range of [H+] and
[OH-] by using logarithms.
• An acid adds [H+] to a solution
• A base reduces [H+] in a solution
so now it has more [OH-]
The product is the pH scale
(potential hydrogen), which
compresses the range of [H+] and
[OH-] by using logarithms.
7 is neutral, below 7 is acidic, above
7 is basic
Buffers minimize changes in [H+] and
[OH-] and are usually a weak acid or
a weak base
Ex: Carbonic Acid acts as a buffer in
our blood.
Acid Rain
• Below pH 5.6
• Caused by a reaction of water
and sulfur oxides and nitrogen
oxides in the atmosphere
• Harmful effects on terrestrial
and fresh water ecosystems
Chemical Reactions
• The process of making and breaking
bonds
• What are the 5 types?
Synthesis
Double Replacement
Single Replacement
Combustion
Decomposition
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