Electron pair repulsion theory
Electrons have a negative charge.
Therefore electron pairs repel each other electron pairs.
This causes the electron pairs to become further apart from each other.
This gives the molecule or ion its shape.
The shape of a molecule or ion is determined by the number of bonded pairs and lone pairs in the outer shell of the central atom.
Molecules with bonded pairs
A dot and cross diagram is used to show the number of electron pairs surrounding the central atom.
Molecules have different shapes when there are different numbers of bonded pairs on the central atom.
The shape and bond angles are a result from the repulsion of electron pairs.
Each bonded pair is repelled as far away as possible from other bonded pairs.
The resulting shapes may be three-dimensional.
Molecules with lone pairs
A lone of pair of electrons is more electron-dense than a bonded pair.
This means that lone pairs repel more than bonded pairs.
Each lone pair reduces the angle by about 2.5 degrees. This is the result of the
extra repulsive effect.
The relative strengths of repulsion are:
lone pair/lone pair > bonded pair/lone pair > bonded pair/bonded pair
The shape of the molecules and bond angles
Bond Angle 180
Number of electrons 8e
Number of electron pairs 4e
Number of lone pairs 0
Bond Angle 104.5 [109.5 - (2.5 x 2)]
Number of electrons
Number of electron pairs
Number of lone pairs 2