Inert gases are very stable and show very few chemical reactions. On observing the electronic configuration of these gases, it was found that in all noble gases (except helium), the outermost shall contains eight electrons (an octet). Helium contains two electrons in the outermost shell.
It was concluded that the chemical stability of noble gases is due to the presence of eight electrons (two in case of helium) in the outermost shell. This electronic arrangement was, therefore, regarded as a stable configuration. In case of all other elements which are chemically reactive, the number of electrons present in outermost shell is less than eight. These elements combine with each other in order to complete their octet and hence, attain a stable configuration.
Thus, the cause of chemical combination is to attain a stable configuration of eight electrons in their outermost shell (octet rule). However, for hydrogen and helium, the stable configuration is of two electrons and is known as duplet. The atoms complete their octets by losing, gaining or sharing electrons.
