Answer: 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.