In
chemistry, a lone pair is a pair of unbonded
electrons orbiting an atom in its outer shell, where the atom itself forms part of a molecule.
Lone pairs have a great significance in the chemistry of many compounds. Due to their negative charge they are strongly attracted to positive charge, so anything with a lone pair can act as a nucleophile - it can attack centres of positive charge.
Lone pairs often play an important role in compounds containing nitrogen or oxygen. Some compounds containing lone pairs are:
When a lone pair attacks a positive charge centre to form a bond between the two molecules, this is known as dative covalent bonding. The bond is
covalent, because electrons are shared, but both the electrons come from the same atom.
When an amine (e.g. methylamine, CH3NH2) reacts with hydrochloric acid (HCl), the lone pair on the nitrogen atom forms a dative covalent bond with the hydrogen atom from the HCl. This creates the methylammonium ion, CH3-NH3+, which reacts with the chloride (Cl-) ion to produce the salt methylammonium chloride, CH3-NH3+ Cl-.
Dative bonding is also present in carbon monoxide (CO), in which one of the three bonds is formed by a pair of electrons from the oxygen atom.
Compounds whose lone pairs form dative bonds with the ions of transition elements are called ligands. The resulting complex ions often have striking colours - for example copper(II) sulphate solution (CuSO4), in which the water acts as a ligand, is deep blue.
Lone pairs also influence molecular shape, because they repel each other quite strongly. In a molecule of water, the hydrogen atoms are pushed together relatively closely to make way for the repulsion between the two lone pairs on the oxygen atom.
It should be remembered that the concept of the lone pair is grounded in covalent bond theory. The equivalent concept in molecular orbital theory is a doubly occupied non-bonding orbital.