Atoms in a molecule are said to be bonded when they share pairs of electrons with each other and this type of bonding is known as covalent bonding. When they share equal number of electrons in a pair and a full electron shell is completed, they become the most stable. But when the electrons are shared unequally by the atoms in a molecule, a POLAR COVALENT BOND is said to be shared by them. This happens as a result of two atoms sharing electron with different electronegativity i.e. the tendency to attract a shared pair of electrons.