State octet rule.

Hint: Octet rule was given by Thumb in the field of chemistry and it mainly originated to those elements which are present in the main group or for those atoms whose atomic number is less than $20$.

Complete answer:
As the name ‘Octet’ suggested that it is clearly related to the number eight. The Octet rule states that main group elements try to bond with other atoms or species in such a way that all of them will possess stable electronic configuration. In other words we can also say that eight electrons in the outermost shell or valence shell of each atom.
Example of compounds which follows the octet rule:
Carbon dioxide (${\text{C}}{{\text{O}}_{\text{2}}}$) also follows the octet rule and bonding structure of carbon dioxide is as follow-
${\text{O = C = O}}$
-In the above structure one oxygen atom binds to the carbon atom through double bond or through two electrons.
-As we know that in the outermost shell of oxygen six electrons are present but after binding with carbon, valence shell electrons of oxygen are eight.
-Similarly in the outermost shell of carbon only four electrons are present but after binding with two oxygen atoms through four electrons, valence shell electrons of carbon are eight.

Note:
Here some of you may count incorrect electrons if you try to count all the electrons which are present in an atom. If you do so then you will never get the correct number of valence shell electrons.