V
S
E
P
R
(VSEPR) T
Chemistry 11
November 7, 2008

V
T
 What is it?
 Model developed to predict the shapes of molecules

 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

VSEPR T
 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

 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

 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 pairbonding pair (LP-BP) repulsion, which in turn is stronger than the bonding pair-bonding pair (BP-
BP) repulsion.

T
R
 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
Domain
 Thus, all are considered to be one electron pair

1.
S
D
VSEPR S
Draw the Lewis structure of the molecule
2.
Determine the central atom (the least electronegative)
3.
Determine the number of bonding pairs
4.
Determine the number of lone pairs
5.
Consult the VSEPR chart to find the shape

HART

Number of
Electron
Groups
2
Name of
Molecular
Shape
Linear
Type of
Electron Pairs
Shape
All Bonding
Pairs
3 Trigonal
Planar
All Bonding
Pairs
4 Tetrahedral All Bonding
Pairs
4
4
Pyramidal 3 Bonding Pairs
1 Lone Pair
Bent 2 Bonding Pairs
2 Lone Pairs
CH
4
NH
3
Example
CO
2
CH
2
O
H
2
0


 As a Lewis Dot diagram:
 Has two electron pairs - two sets of double bonds – two bonding pairs
 Creates a LINEAR shape according to VESPR


 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


 As a Lewis Dot diagram:
 Has four electron pairs - four bonding pairs and no lone pairs
 Creates a TETRAHEDRAL shape according to
VESPR


 As a Lewis Dot diagram:
 Has four electron pairs - three bonding pairs and one lone pair
 Creates a PYRAMIDAL shape according to VESPR


 As a Lewis Dot diagram:
 Has four electron pairs - two bonding pairs and two lone pairs
 Creates a BENT shape according to VESPR

Practice
 Draw the VSEPR shape for each of these molecules
 HCN
 CCl4
 SI2
 H3O +
 CH2F2
 AsCl3
 NO2 +
 CH3F
 NF3
 CCl2F2
 NCl3
 SO4 2-
 COCl2
 NO2 -
 NH4 +
Finished?
Assign VSEPR shapes to the homework sheet (questions 1-
5) from last class. 