CHM 123
Chapter 7
7.9 Molecular shapes and VSEPR theory
VSEPR theory proposes that the geometric arrangement of terminal atoms, or groups of atoms
about a central atom in a covalent compound, or charged ion, is determined solely by the
repulsions between electron pairs present in the valence shell of the central atom
In the valence-shell electron-pair repulsion theory (VSEPR), the electron groups around a
central atom
• are arranged as far apart from each other as possible.
• have the least amount of repulsion of the negatively charged electrons.
• have a geometry around the central atom that determines molecular shape.
Electrons in bonds and in lone pairs can be thought of as “charge clouds” that repel one another
and stay as far apart as possible, this causing molecules to assume specific shapes.
Working from an electron-dot structure, count the number of “charge clouds,” and then determine
the molecular shape.
The following are the “parent” electronic structures upon which VSEPR is based. These structures
show how to minimize the energy of the structure by placing 2, 3, 4, 5 or 6 electron groups (charge
clouds) as far apart around a central atom as possible in three-dimensional space.
Dang 1
How many electron groups surround the central atom in the Lewis formulas?
CH4
NH3
H2O
In each of these examples, the
electron pairs are arranged
tetrahedrally, and two or more
atoms are bonded in these
tetrahedral directions to give the
different geometries.
Lone pairs are absolutely critical
part of the electronic structure
(charge clouds) that contributes to
the shape of the molecule, but
only the attached atoms are
included in deriving the shape
name.
How many electron groups (charge clouds) are around the central atom in the following?
Dang 2
Dang 3
Total # of
e- groups
on central
atom
2
3
3
4
4
4
5
“Parent” electronic
geometry
#
Lone
pairs
Idealized
molecular shape
Idealized
bond angles
Linear
Trigonal Planar
Trigonal Planar
Tetrahedral
Tetrahedral
Tetrahedral
Trigonal Bipyramidal
#
Bond
ed
atoms
2
3
2
4
3
2
5
0
0
1
0
1
2
0
Linear
Trigonal Planar
Bent
Tetrahedral
Trigonal Pyramidal
Bent
Trigonal Bipyramidal
5
Trigonal Bipyramidal
4
1
Seesaw
5
5
6
6
6
Trigonal Bipyramidal
Trigonal Bipyramidal
Octahedral
Octahedral
Octahedral
3
2
3
0
1
2
T-shaped
Linear
Octahedral
Square Pyramidal
Square Planar
180o
120 o
120 o
109.5 o
109.5 o
109.5 o
90 o, 120 o,
180 o
90 o, 120 o,
180 o
90 o, 180 o
180 o
90 o, 180 o
90 o, 180 o
90 o, 180 o
2
6
5
4
Examples: Name the shape and give the idealized bond angles for the following
Lewis structure
Shape
Idealized bond angle
Dang 4
Real bond angle
Real bond angles vs. Idealized bond angles
VSEPR predicts the idealized bond angle(s) by assuming that all electron groups take up the same
amount of space. Since lone pairs are attracted to only one nucleus, they expand into space further
than bonding pairs, which are attracted to two nuclei. As a result, real molecules that has lone pairs
on the central atom often have bond angles that are slightly different than the idealized prediction.
Central atom without lone pairs has
the same real bond angle as the idealized
angle.
The exceptions to this are square
planar shapes and linear (derived from
trigonal bipyramidal electronic structure)
shapes where the lone pairs offset one
another, thus causing no deviation from
ideality.
Molecules with no central atom
Many molecules don’t have a “central” atom
but many “central” atoms. These molecules
don’t fit into the shape names that we’ve
learned. However, we can give an
approximated shape and bond angle to each
“central” atom at a time.
Example: Give the approximate shape at the
numbered “central” atoms
Drawing 3-D structures
In order to draw 3-D structure, chemists use dark wedges to
indicate a bond projecting forward (out of the page) and dashed
to indicate a bond going away (going back into the page) and a
normal line to indicate a bond in the plane of the page
Examples: Draw 3-D structure for the following molecules
Dang 5