Resonance Structures, Formal Charge, VSEPR Models
1. Resonance Structures- occurs when more than one valid Lewis structure can be written for a
particular molecule. The resulting electron structure is an average of the resonance structures.
2. Odd electron molecules- odd number of electrons. Such as NO cannot be handled by the LE
Model.
3. Formal Charge- A method that allows you to choose the correct Lewis Dot Structure when
more than one valid structure exists.
A. Defined as the difference between the number of valence electrons on the free atom and the
number of valence electrons assigned to the atom in the molecule.
1. Find the number of valence electrons on the free neutral atom.
2. Find the number of valence electrons belonging to the atom in the molecule.
B. To calculate formal charge:
Formal charge = (number of electrons on free atom) – (number of valence electrons assigned to
the atom in the molecule)
C. If you make these assumptions to calculate valence electrons assigned to the atom in the
molecule.
1. Lone pair electrons belong entirely to the atom in question.
2. shared electrons are divided equally between two sharing atoms.
(valence electrons assigned to the atom in the molecule) = (# of lone pair) + ½(# of shared
electrons).
The formal charge closest to Zero is the correct one.
4. VSEPR
A. VSEPR stands for Valence Shell Electron Pair Repulsion Theory.
B. Based on the repulsion of electrons in the bonding atoms. They want to be as far away as
possible so the shape of the molecule ensures the greatest distance.
C. How to determine the Shape
a. Draw a Lewis Dot Structure.
b. Count the number of shared electrons (bonding pairs) also called Sigma Bonds
c. Count the number of Lone Pairs.
d. Look up the shape on your Second Best Friend.
# of electron pairs
around central
atom
2
# of sigma bonds
(bonding pairs)
# of lone pairs
Shape around central atom
Angle
2
0
Linear
180
3
3
0
Trigonal planer
120
4
4
0
Tetrahedral
109.5
4
3
1
Trigonal pyramidal
107.5
4
2
2
Angular or bent
104.5
4
1
3
Linear
180
5
5
0
Trigonal bipyramidal
90 & 120
6
6
0
Octgahedral
90