Electron Shell Rules;
1) Electrons always occupy shells (sometimes called energy levels).
2) The lowest energy levels are always filled first.
3) Only a certain number of electrons are allowed in each shell:
1st shell = 2. 2nd shell = 8. 3rd shell = 8.
4) Atoms are much happier when they have full electron shells.
5) In most atoms the outer shell is not full and this makes the atom want to react.
6) An element's electron arrangement determines its chemical properties.