Duplet rule

The octet rule is a chemicalrule of thumb that describes the strong tendency for main-group elements to bond in such a way that each atom has eight electrons (an "octet") in its valence shell, giving it the same electronic configuration as a noble gas atom. Electrons shared between two atoms in a covalent bond, are counted toward the octet of each atom. The rule is especially applicable to carbon, nitrogen, oxygen, and the halogens, although more generally the rule is applicable for the s-block and p-block of the periodic table. Other rules exist for other elements, such as the duplet rule for hydrogen and helium, and the 18-electron rule for transition metals.