What is the duet rule?
The duet rule states that hydrogen and helium may have no more than two electrons in their valence shells.
The rule comes from quantum mechanics, which says that the lowest energy level (n = 1) of an atom can contain only two electrons.
The duet rule applies to H and He, because they are the first two atoms in the Periodic Table.
The next energy level (n = 2) can contain up to eight electrons. This gives rise to the octet rule.