The tendency of atoms of elements to gain or lose electrons so that their outer s and p orbitals are filled with eight electrons is called rule

Octet rule

Explanation: The elements which have filled s and p orbitals or noble gases are most stable and thus every element tends to get stable by completing their octet.

For example:  Sodium [Na] has atomic no of 11 and thus the electronic configuration is : tends to get stable by losing one electron and gets converted to

Electronic configuration for  

Thus acquiring stable configuration with filled s and p orbitals.

For example:  Fluorine [F] has atomic no of 9 and thus the electronic configuration is : tends to get stable by gaining one electron and gets converted to

Electronic configuration for

Thus acquiring stable configuration with filled s and p orbitals.