Molecular Geometry, using Lewis Structures, the VSEPR and Valence Bond Theories
These notes should not replace the reading of good text books.
Physical and Chemical properties depend on the geometry of a molecule.
Molecular Geometry (three dimensional structure) depends on the nature of the central atom
(with or without d-orbitals): : the bonding electrons and the lone pairs around it.


Use Lewis structures and the VSEPR theory: Valence Shell Electron Pair Repulsion,
bonding electrons and lone pairs (the valence electrons) are placed on a sphere as far
apart as possible
Use Lewis structures and the Valence bond (VB) theory : Bonding electrons and lone
pairs are accommodated in hybridized orbitals, as far apart as possible in three
dimensional space
For Molecular Geometry (Structure): the positions of the atomic nuclei will determine
the geometry
BP = Bonding Pairs, LP = Lone Pairs
Electron Pair Geometry: AX2(2 BP)
Molecular Geometry: Linear
Examples
Electron Pair Geometry: AX3 (3BP or 2BP + 1LP)
Molecular Geometry: AX3 Trigonal Planar Molecular Geometry: AX2E1 Bent/Angular
Examples
Examples
Electron Pair Geometry: AX4 [4BP or (3BP + 1LP) or (2BP + 2LP)]
Molecular Geometry: AX4 Molecular Geometry: AX3E1 Molecular Geometry: AX2E2
Tetrahedral
Trigonal Pyramidal
Bent/Angular
Examples
Examples
Examples
Electron Pair Geometry: AX5 [5BP or (4BP + 1 LP) or (3BP + 2LP) or (2BP + 3LP)]
Molecular
Geometry:
AX5
Trigonal
Bipyramidal
Molecular
Geometry:
AX4E1
See-saw
Molecular
Geometry:
AX3E2
T-structure
Molecular
Geometry:
AX2E3
Linear
Examples
Examples
Examples
Examples
In AX5: More electronegative atoms in the axial positions, (bond will be a bit longer), and lone pairs
and double bonds in the equatorial position, see see-saw, T- and linear-structures.
Electron Pair Geometry AX6 [6 BP or (5BP + 1 LP) or (4BP + 2LP)]
Molecular Geometry: AX6 Molecular Geometry: AX4E1 Molecular Geometry: AX4E2
Octahedral
Pyramidal Planar
Square Planar
Examples
Examples
Examples
To predict molecular geometry, find the nuclei of the atoms in three dimensional space, after
defining the electron pair geometry from eg AX3E2 as AX3+2 = AX5, remember the distortions.
Distortions in bond angles are influenced by (1) the lone pairs on the central atom and (2) the size
of atoms, eg
H2O
OF2
OCl2
HOH 104.5 °
FOF 103 °
ClOCl 111 °
With lone pairs on the central atom, the bond angle will not be the AX4 109.5 °.
The HOH bond will be smaller than the standard 109.5 °, because of the larger volume of the two lone
pairs on the oxygen atom, but in OF2 the more electronegative F atoms will draw the lone pairs closer
to the OF single bonds, influencing the bond angle more. The ClOCl bond angle is measured as 111 °,
larger than the expected 109 °, because of the larger chlorine atoms, they move away from each other,
repulsion of the electronic charges on the large atoms.