1. Valence Shell Electron Pair Repulsion (VSEPR) Theory
Chemistry 11
November 7, 2008

2. Vsepr Theory Remember… What is it?
Model developed to predict the shapes of molecules
Remember…
Atoms are bound together by electron pairs called bonding pairs
These can be single (one pair e- = single bond) or multiple (2 pair e- = double bond; 3 pair e- = triple bond)
Some atoms in a molecule can also have pairs of electrons not involved in bonding called lone pairs or non-bonded pairs

3. VSEPR Theory Why are we concerned with the shape of molecules?
Will help us later when we discuss polarity
What causes molecules to take on a different shape?
Electron pairs (bonding pairs and lone pairs) are negatively charged and will repel each other
These pairs of electrons tend to occupy positions around the atom that minimize repulsions and maximize the distance of separation between them

4. Three types of repulsions take place in an atom
Lone Pair – Lone Pair (LP-LP)
Lone Pair – Bonding Pair (LP-BP)
Bonding Pair – Bonding Pair (BP-BP)
Lone pairs occupy more space than bonding electron pairs
Double bonds occupy more space than a single bond

5. LP-LP > LP-BP > BP-BP
A molecule must avoid these repulsions to remain stable.
When repulsion cannot be avoided, the weaker repulsion (i.e. the one that causes the smallest deviation from the ideal shape) is preferred.
Lone pair-lone pair (LP-LP) repulsion is considered to be stronger than the lone pair- bonding pair (LP-BP) repulsion, which in turn is stronger than the bonding pair-bonding pair (BP- BP) repulsion.
LP-LP > LP-BP > BP-BP

6. Things to Remember
When assigning a VSEPR Shape to a molecule, we focus on the central atom and the bonding pairs or lone pairs associated with it
Ex. CO2
Electron pairs are considered to exist in a domain
Domains can be made up of:
A lone pair
A single bond
A double bond
A triple bond
Thus, all are considered to be one electron pair
Domain

7. Steps to Determine VSEPR Shape
Draw the Lewis structure of the molecule
Determine the central atom (the least electronegative)
Determine the number of bonding pairs
Determine the number of lone pairs
Consult the VSEPR chart to find the shape

8. VSEPR Chart

9. Number of Electron Groups
Name of Molecular Shape
Type of Electron Pairs
Shape
Example 2
Linear
All Bonding Pairs
CO2 3
Trigonal Planar
CH2O 4
Tetrahedral
CH4
Pyramidal
3 Bonding Pairs
1 Lone Pair
NH3
Bent
2 Bonding Pairs
2 Lone Pairs
H20

10. Carbon Dioxide CO2 As a Lewis Dot diagram:
Has two electron pairs - two sets of double bonds – two bonding pairs
Creates a LINEAR shape according to VESPR

11. Methanal CH2O As a Lewis Dot diagram:
Has three electron pairs - three bonding pairs and no lone pairs
Note: Only has three bonding pairs because the double bond is considered to be one bonding pair!
Creates a TRIGONAL PLANAR shape according to VESPR

12. Methane CH4 As a Lewis Dot diagram:
Has four electron pairs - four bonding pairs and no lone pairs
Creates a TETRAHEDRAL shape according to VESPR

13. Ammonia NH3 As a Lewis Dot diagram:
Has four electron pairs - three bonding pairs and one lone pair
Creates a PYRAMIDAL shape according to VESPR

14. Water H2O As a Lewis Dot diagram:
Has four electron pairs - two bonding pairs and two lone pairs
Creates a BENT shape according to VESPR

15. Practice Draw the VSEPR shape for each of these molecules HCN CCl4 SI2
CH2F2
AsCl3
NO2+
CH3F
NF3
CCl2F2
NCl3
SO42-
COCl2
NO2-
NH4+
Finished?
Assign VSEPR shapes to the homework sheet (questions 1-5) from last class. 