Shapes of Molecules

The shape of all molecules / ions are dictated by the number and type of electron pairs present in the molecule according to the Valence Shell Electron Pair Repulsion theory (VSEPR).

The distribution of electron pairs around a central atom will dictate the shape of simple molecules. Electron pairs can either be bonding pairs ((b.p.), i.e. a typical covalent bond) or lone pairs (l.p.) of electrons. Lone pairs of electrons occupy orbitals that sit closer to the central atom than bonding pairs (which occupy molecular orbitals shared between two atoms); this means that they take up more space around the central atom.

To work out shape of molecule, just note the number and type of electron pairs around the central atom. The shape of molecule will be one of the following:

Number and type of electron pair - shape - bond angle 

1 b.p.  - linear - 180

2 b.p. - linear - 180

3 b.p.  - trigonal planar - 120

2 l.p. & 2 b.p  - v-shaped  - 104.5

3 b.p. 1 l.p. -  pyramidal - 107

4 bonding pairs - tetrahedral - 109.5

5 bonding pairs  - trigonal bipyramidal - 90 & 120 (not on lc course

6 bonding pairs - octahedral -90 (not on lc course)

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