The chemical bond formed between two atoms due to unequal sharing of electrons is        

In hetero nuclear molecule, the bond pair electrons sharing to be unequal between the bonded atoms. This is due to difference in the electronegative values of bonding atoms in a covalent bond. As a result the bond pair electrons displaced closely towards the atom which has more electronegative than other atom. Now, there the covalent bond gets polarization that is cation (partial positive charge) and anion (partial negative charge). This type bond is named as a polar covalent bond.

Ex: HCl

$H^{\delta +}\mathrm{......}C{l}^{\delta -}$