VSEPR, MOLECULAR GEOMETRY

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 Tetrahedral	Molecular Geometry: AX₃E₁ Trigonal Pyramidal	Molecular Geometry: AX₂E₂ 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 Octahedral	Molecular Geometry: AX₄E₁ Pyramidal Planar	Molecular Geometry: AX₄E₂ 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.