Chapter 2 (CIC) and
Chapter 8 (CTCS)
• Read in CTCS Chapter 8.1, 4, 6-8
• Problems in CTCS: 3, 31, 33, 47abcdf,
49bd, 51a, 59bd, 61
Ozone
• A pollutant in troposphere
• A filter of UV light in stratosphere
• 3 O2(g) + Energy  2 O3(g)
Energy = lightning, photocopier, electric arc, etc.
• Allotrope (e.g., graphite, diamond, fullerene)
• Used for bleaching (wood, fabric, water)
• Why is ozone different from oxygen and why is
it helpful in one part of our atmosphere but not
in another?
Periodic Table
• Why is periodic table laid out the way it is?
• Li, Na, K demo
• Valence electrons (e-) – account for chemical
and physical properties of elements and are
the outermost electrons of an atom
• Can be determined for any representative
element by the number above the Group
(Family) in the periodic table (1A-8A)
• Families: Alkali metal, Alkaline Earth,
Chalcogen, Halogen, Noble Gas
Getting to Low Energy
• Noble Gas – “inert” because it is stable by itself
• These elements have 8 valence e- (except He)
• Other elements try to attain this low energy
state as well
• Elements do this by gaining, losing, or sharing
e• Sharing of e- yields covalent bonds
Lewis Structures
• Lewis symbols
H
Li
He
Be
B
C
N
O
F
Ne
• Two H· atoms can share their electron so
that each looks like He (noble gas)
H
H
H
H
• The line between H’s is shorthand for 2 eand is called a single bond
• In the case of fluorine gas (F2), each atom can
share an e- so that at any point in time, either atom
can look like neon (It follows the Octet Rule)
F
F
F
F
• This structure has both bonding and nonbonding epairs
Q: Draw Lewis structures for Cl2 and CH4
Multiple Bonds
• For a Lewis structure of O2 each O needs two eto supplement the 6 valence e- of oxygen
• This requires a sharing of 4 e- total
O
O
O
O
O
O
• Oftentimes the nonbonding e- on Lewis structures
are not shown
• This molecule has a double bond
• These are shorter, stronger and harder to break
Rules for Lewis Structures
1. Count the number of valence e2. Draw a structure where the peripheral atoms
surround the central atom
a. The central atom is usually the first atom written (S
in SO42-, I in IO65-)
b. The central atom is usually the most metallic
element
c. H can never be the central atom because it can only
have a duet meaning one bond
d. When H and O exist in the same molecule, the H is
usually attached to the O atom (H2CO3)
3. Draw covalent (or shared) bonds between the
peripheral atoms and the central atom
4. Determine the number of valence e- still
available
5. Fill the octets for the peripheral atoms.
6. Fill the octet for the central atom (if there are
enough valence e-)
7. If there are not enough e- to accomplish #6,
make multiple bonds between the central and
peripheral atoms
You should check two things when finished:
1. Are you showing the correct number of valence
e-?
2. Does each atom have an octet?
Draw Lewis Structures for CO2, N2, CO, PCl3,
CH3OH and H2CO
It doesn’t matter what the bond angles are
The structure of O3
1.
3 x 6 = 18e2,3. O-O-O
4.
18 - 4 = 14e5.
O O O
6.
O
O
O
7.
O O
O
*These are not linear molecules
O
O O
Resonance
O O
O
O
O O
• These two structures are averaged together
to get the “actual” structure and they are
referred to as resonance structures
• The actual structure has an O-O bond length
of 1.28Å compared to an O-O typically at
1.47Å and O=O at 1.21Å
• Resonance structures only move e-, not
atoms
Formal Charge
• Write a Lewis Structure for Cl2CO
• How do you know which one is closer to reality?
• F.C. = #valence e- - #lone pair e- - #bonds
– The total sum of formal charges should be equal to the charge on the
ion/molecule
– You should end up with a Lewis structure that has formal charges as close to 0 as
possible
Exceptions to Octet Rule
• Write a Lewis structure for NO
• Many exceptions exist
– Odd numbered electron systems
– Electron deficient atoms (Be, B, Al, etc.)
– Central atom has more than 8 electrons (P, S, I, etc.)
• Write a Lewis structure for BF3 and calculate
formal charges. How does it react with NH3?