Atoms of elements combine to form molecules of compounds. This combination can occur in two ways: either by electrovalent bonding or covalent bonding. In all chemical reactions it is the electrons from the outermost shell of an atom that are involved in interacting with other atoms, either by their transfer or by sharing.
The atoms of all elements except noble gases, have incomplete outermost shell and hence are reactive. They try to achieve the stability of the inert gases by electron transfer or by electron sharing. After this combination, the molecule becomes much less reactive. Noble gases have their outermost shell complete and hence they are unreactive or "inert".
