Suppose that a molecule has four bonding groups and one lone pair on the central atom. Suppose further that the molecule is confined to two dimensions. (This is a purely hypothetical assumption for the sake of understanding the principles behind VSEPR theory.) Estimate the bond angles between electron groups.

The bond angles between the axial bonding groups are slightly less than 180°.

The bond angles between the equatorial bonding groups are slightly less than 120°.

Explanation:

Accordign to VSEPR theory, a molecule with four bonding groups and one lone pair on the central atom has a trigonal bipyramidal electronic geometry.

The position of the lone pair can be located in the equatorial position or axial position.

When the lone pair is found in equatorial position, it has two axial groups that repel it and the angle of the lone pair between each axial group is 90°.

When the lone pair is in axial position it has 3 equatorial groups that repel it and the angle of the lone pair between each equatorial group is 90°.

Since the molecule has a lone pair, the most stable geometric structure is when the lone pair is in the equatorial position, because it has fewer repulsions than in the axial position.

The molecular geometry is "seesaw"

The bond angles between the axial bonding groups are slightly less than 180°.

The bond angles between the equatorial bonding groups are slightly less than 120°.