The duet rule in chemistry is an adaptation of the octet rule that applies to hydrogen and lithium. Because atoms of these elements only possess s orbitals, two electrons can create a filled energy state like the nearest noble gas, helium.
According to the octet rule, atoms are most stable when they form bonds and acquire a full shell of electrons in a configuration like that of the nearest noble gas. For most atoms, a set of eight electrons is the goal. However, just as hydrogen and lithium are exceptions with fewer electrons, others can have more, such as bromine and phosphorus with 10 electrons.