Valence
Shell
Electron
Pair
Repulsion
Theory
Planar
triangular
Tetrahedral
Trigonal
bipyramidal
Octahedral
VSEPR Theory
• Based on Lewis structures we can know the
shape or “geometry” of molecules
• VSEPR (pronounced “vesper”) stands for
Valence Shell Electron Pair Repulsion
• VSEPR, as the name suggests, predicts
geometry based on the repulsion of electron
pairs (in bonds or by themselves)
• Electrons around the central nucleus repel
each other. Thus, resulting structures have
atoms maximally spread out
VSEPR overview
• 2 Principles of the Theory
– Electron pairs repel each other
– 2 kinds of pairs
• Bonded pairs (shared between atoms)
• Lone pairs (NOT shared)
• Molecules are represented by AXY, where A
is the central atom and Y is the # of
peripheral atoms
• AX2 = linear
• AX3 = planar triangular
• AX4 = tetrahedral (tetra = 4 faces)
• AX5 = trigonal bipyramidal (2 pyramids)
• AX6 = octahedral (octa = 8 faces)
Planar triangular
Tetrahedral
Trigonal bipyramidal
Octahedral
Lone pairs
• Thus far we have considered (built) only
structures where there are no free electrons
around the central atom
Vs.
H
H
or
or
H N H
H N H
H C H
H C H
H
H
H
H
• These electrons that are not involved in bonds are
called “lone pairs”
• Essentially, they have the same influence on
molecular structure as electron pairs in bonds
• The result is some weird shapes and names…
Variations on Tetrahedral Molecule
• The tetrahedral molecule is AX4
• Lone pairs can be indicated with
AXYEZ, where Z is the number of
lone pairs
• By replacing 1 bond with a lone pair
the tetrahedral shape becomes
“trigonal pyramidal”
• AX3E
• By replacing two bonds with lone
pairs we get a “bent” (non-linear)
shape (AX2E2 ) (Fig. 3.26, pg. 87)
Variations on Trigonal Bipyramidal
• AX5 is trigonal
bipyramidal
• AX4E is
unsymmetrical
tetrahedron
• AX3E2 is Tshaped
• AX2E3 is
linear
See Fig. 3.37
on pg. 91
Variations on Octahedral Shape
• AX6 is octahedral
• AX5E is
square
pyramidal
• AX4E2 is square
planar