What is lone pair effect?

Answer: A lone pair is an unshared pair or non-bonding pair of valence electrons that do not exchange places with another atom during the formation of a covalent bond. 

Lone pairs are present in the outermost electron shell of atoms. They could be specified through the use of a Lewis structure.

For example,

The lone pair effect occurs when an unshared pair of electrons is completely shared by another atom or ion around an atom in the middle of a molecule. 

The lone pair effect occurs in polar covalent substances like NH₃ and HCl.