The tendency of atoms to complete their outer energy level is known as the octet rule. The octet rule states that atoms below atomic number 20 tend to combine so that they each have eight electrons in their valence shells, which gives them the same electronic configuration of a noble gas. This rule is applicable to what are termed as main- group elements, especially carbon, nitrogen, oxygen, and the halogens but also metals such as magnesium and sodium, among others.