A pair of electrons which is not shared by any of the reacting atoms is called a lone pair of electrons. Lone pairs are found in the outermost electron shell of atoms. They can be identified by using a Lewis structure.

The structures of the compounds are as follows,

Hence the compounds ${\mathrm{NH}}_3,{\mathrm{O}}_3,{\mathrm{SF}}_4 \mathrm{and} {\mathrm{BrF}}_5$ have one lone pair on central metal atom.