Why a dipole develops in a molecule?
Sharing of a pair of electrons between two atoms forms a covalent bond. The two atoms involved in bond formation may be the same (homonuclear), or different (heteronuclear). Owing to the difference in electronegativities, the shared pair of electrons in covalent bonds are not shared equally. The electron pair resides nearer to the atom having greater electronegativity, thus forming a negative bias towards one atom. In other words, a negative ‘pole’ is created somewhere along the bond. Another counter pole of exactly same magnitude is formed simultaneously, but in opposite direction. Thus, a dipole is created. Its magnitude is expressed as a moment, the dipole moment, which is the product of extent of charge separation and their intervening distance.