When there are lone pairs on the valence shell of the central atom in a molecule, the bond angle is decreased.
The shape of molecules depend on the number of electron pairs on the valence shell of the molecules. These electron pairs could be lone pairs or bond pairs. Lone pairs occupy more space around the central atom since it is attracted to only one nucleus compared to bond pairs which are attracted to two nuclei.
When lone pairs are present on the central atom in a molecule, they tend to "squeeze" the molecule thereby decreasing the bond angle in the molecule. Hence, presence of one or more lone pairs of electrons make the bond angles get smaller.
