VALENCE SHELL ELECTRON PAIR
REPULSION (VSEPR) THEORY
VSEPR THEORY
 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
VSEPR THEORY
 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
bond
bonds occupy more space than a single
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 (BPBP) repulsion.
LP-LP > LP-BP > BP-BP
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
STEPS TO DETERMINE VSEPR SHAPE
1.
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
VSEPR CHART
Number of
Electron
Groups
Name of
Molecular
Shape
Type of
Electron Pairs
Shape
Example
 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
 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

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
 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

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
Practice
Predict and draw the VSEPR shape for each of these
molecules:
 HCN
 NF3
 CCl4
 CCl2F2
 SI2
 NCl3
 AsCl3
 COCl2
