The octet rule says atoms are most stable energetically when the valence shell holds:

1. Chemical behaviour is set by the outermost electron shell. 2. The innermost shell has a maximum of two valence electrons but the next two electron shells can each have eight. 3. The octet rule states, with the exception of the innermost shell, that atoms are more stable energetically when they have eight electrons in their valence shell. 4. Two electrons is the special case of the innermost shell only. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 2 'The Chemical Foundation of Life', sections 2.1-2.3_