November 20, 2009
•C h a p t e r 8
•Second
set of Chapt. 8 homework dates extended
until after break
•Lab Notes
•Today’s Topic:
Electron Distribution in Molecules (8.5)
Molecular Shape (8.6)
Molecular Polarity (8.7)
Formal Charge and Oxidation Number Examples
Partial Charges
Charges Within Molecules
 Formal charge
Electrons are shared evenly throughout molecule
 Closer to “real” charge than oxidation number
 Use to determine which nonequivalent resonance structure is more
likely
 Oxidation Number
 Bonding electrons “belong” to more electronegative atom
 Quick and easy to determine without Lewis structure
 Doesn’t really reflect actual charges on atoms
 Use to determine whether compounds are oxidized/reduced in
chemical reactions
 Partial Charges
 How electrons are really distributed in a molecule
 Need computer to calculate
 Used only when you need very accurate description- molecular
modeling

What is the formal charge on C in CN-?
1. -2
63%
2. -1
3. 0
4. +1
5. +2
12%
8%
1
2
3
6%
4
10%
5
What is the formal charge on C in CN-?
1. -2
20%
20%
20%
20%
20%
2. -1
3. 0
4. +1
5. +2
1
2
3
4
5
Molecular Structure
Chemical
Formula
Lewis
Structure
#Structural
Pairs
(Valence ePairs)
Electron
Pair
Geometry
Molecular
Geometry
Lewis
Structure
# Structural
Pairs
# Structural Pairs = # bonded
atoms
Electron
Geometry
+
# lone
pairs
Molecular
Geometry
on central atom
# Structural Pairs determines Electron Pair Geometry
# Structural Pairs determines Electron Pair Geometry
# Structural Pairs determines Electron Pair Geometry
What is the electron pair geometry of ozone?
1.
2.
3.
4.
5.
Linear
Trigonal planar
Tetrahedral
Trigonal bipyramidal
octahedral
57%
21%
13%
9%
0%
1
2
3
4
5
What is the electron pair geometry of SCN-?
1.
2.
3.
4.
5.
Linear
Trigonal planar
Tetrahedral
Trigonal bipyramidal
octahedral
77%
13%
9%
2%
1
2
3
4
0%
5
Molecular Geometry = Electron Pair Geometry, ignoring lone pairs
BF3 and O3
Molecular Geometry = Electron Pair Geometry, ignoring lone pairs
Molecular Geometry = Electron Pair Geometry, ignoring lone pairs
Lone pairs always
go in equatorial sites
Molecular Geometry = Electron Pair Geometry, ignoring lone pairs
What is the molecular geometry of NO2-?
1.
2.
3.
4.
5.
6.
7.
8.
9.
Linear
Bent
Trigonal planar
Tetrahedral
Trigonal pyramid
T-shaped
Square pyramid
Square planar
octahedral
11%
11%
1
2
11%
11%
11%
11%
11%
3
4
5
6
7
11%
11%
8
9
What is the molecular geometry of …
1.
2.
3.
4.
5.
6.
7.
8.
9.
Linear
Bent
Trigonal planar
Tetrahedral
Trigonal pyramid
T-shaped
Square pyramid
Square planar
octahedral
11%
11%
1
2
11%
11%
11%
11%
11%
3
4
5
6
7
11%
11%
8
9
What is the molecular geometry of …
1.
2.
3.
4.
5.
6.
7.
8.
9.
Linear
Bent
Trigonal planar
Tetrahedral
Trigonal pyramid
T-shaped
Square pyramid
Square planar
octahedral
11%
11%
1
2
11%
11%
11%
11%
11%
3
4
5
6
7
11%
11%
8
9