Rules for counting the number of electrons that give rise to stable structures in atoms, molecules, and solids. A simple example of an electron counting rule is the result that there are 2n² electrons in a fully occupied electronic shell in an atom, where n is the principal quantum number associated with that shell. Other examples of electron counting rules are the Hückel rule, Wade’s rules, and the Jemmis mno rules. Electron counting rules have also proved useful in understanding the structure and stability of alloys.