What is Octet rule ? Briefly explain its significance and limitations.

Explanation

Octet Rule : In order to attain stability, "Atoms show tendency to have 8 electrons in their outermost shell".
Significance of octet rule : It provides an explanation to the question, 'why do atoms combine ?'. It also explains why some atoms are chemically inert. It is able to explain, the cause of formation of different types of bonds, ionic and covalent. The main points of the octet theory are :
1) Atoms with 8 electrons in the outermost shell (2 for He) are chemically stable. So they do not take part in chemical reactions.
2) An atom with less than 8 electrons in outermost shell is reactive and so combines with other atoms. Atoms with less than 4 electrons in the outermost shell tend to lose them, while those having more than 4 electrons in the outermost shell tend to gain the electrons, so that they get the nearest inert gas configuration.
3) Atoms enter into chemical reactions either by losing, by gaining or by sharing electrons with other atoms. This gives rise to different types of chemical bonds.
4) In this process of bond formation, every atom tries to acquire 8 electrons in its outermost shell.
5) The chemical reactivity of an atom is measured by its tendency to lose, gain or shared electrons.
Limitations :
1) Octet rule couldn't explain the shape of molecule.
2) Octet rule is not satisfied for molecules having odd number of electrons.
Ex : No.
3) It could not explain the stability of molecules in which central atom has less or more than 8 electrons.
Ex : In `BeCl_(2)`, Be has 4 electrons in its valence shell.