Lewis Structures, Molecular Shape and VSEPR
The Valence Shell Electron Pair Repulsion
theory (VSEPR) allows one to predict the shape
of a molecule from the Lewis electron dot
structure. The theory is based on the assumption
that the electron pairs surrounding the central
atom repel each other and distance themselves in
the surrounding space with an arrangement
which reduces the amount of repulsions. Such
spatial arrangements can be predicted from a
knowledge of the number of electron groups
surrounding a central atom. The table at the right
summarizes the results of the theory.
When determining the number of electron
groups, a single bond, a multiple bond and a
non-bonding pair are all counted as a single
electron group. The electron pair geometry about
the central atom is directly effected by the
number of electron groups. The molecular shape
is determined by the arrangement of bonded
atoms about the central atom of the molecule. If
all electron groups are involved in bonding, then
the molecular shape and the electron pair
geometry are identical. When an electron group
is a non-bonding pair, the molecular shape can be
determined by visually removing an atom from
one of the corners of the electron pair geometry.
Student mastery of VSEPR is dependent upon:
• an ability to draw Lewis structures
• an understanding of the table at the right
and a willingness to use it
• an ability to spatially visualize geometric
arrangements
• a willingness to use your noodle
• patience and hard work
From Zumdahl, Chemistry, 6th edition
For the following listed compounds, draw the Lewis Dot structure in order to determine the number of electron groups, the number of bonding
and non-bonding groups and the resulting electron pair geometry and molecular geometry. Finally, approximate the value of the bond angles.
Comp'd
1.
CH4
2.
BF3
3.
N2O
(NNO)
4.
Br2
5.
SF4
6.
HCl
Lewis Dot Structure
# of eGroups
# of Bonding
# of NonPairs
Bonding Pairs
e- Pair
Geometry
Molecular
Geometry
~ Bond
Angles
Polarity
Comp'd
7.
PH3
8.
H 3O +
9.
SO3
10.
CO2
11.
XeCl2
12.
SO2
Lewis Dot Structure
# of eGroups
# of Bonding
# of NonPairs
Bonding Pairs
e- Pair
Geometry
Molecular
Geometry
~ Bond
Angle
Polarity
Comp'd
13.
SF6
14.
PCl5
15.
SeO2
16.
ClO2-
17.
ClF2-
18.
O3
Lewis Dot Structure
# of eGroups
# of Bonding
# of NonPairs
Bonding Pairs
e- Pair
Geometry
Molecular
Geometry
~ Bond
Angle
Polarity