Atoms, except those of noble gases, do not have free existence. They readily combine with one another to form molecules. According to the classical view, the phenomenon of combination, called chemical bonding, “involves the union of two or more atoms through redistribution of electrons in their outermost shells involving either transference of electrons from one atom to another or sharing of electron amongst themselves so that all the atoms acquire the stable noble gas configurations of minimum energy”.

1. If the union takes place by the transference of electrons from one atom to another, the bonding is said to be ionic.

2. If it takes place by the mutual sharing of electrons between the atoms concerned, the bonding is said to be covalent.

Note: There is yet another type of bonding called coordinate covalent bonding in which both the electrons shared between that atoms are contributed by one atom only.