Valence electrons

Electrons occupy layers around the nucleus called shells. For the main-group examples here, the electrons in the outermost shell are the valence electrons involved in bonding. Transition metals can also use electrons from an inner d subshell.

Many main-group atoms form stable arrangements with a full outer shell (often 8 electrons — the octet rule, or 2 for hydrogen and helium). Atoms bond when the resulting arrangement is lower in energy. Gaining, losing or sharing electrons often gives the octet pattern, but it is a useful rule with exceptions, not a universal cause of bonding.