The duplet rule is a chemical rule that states that an element is stable if its atom has two electrons in its valence shell, which is the outermost shell of an atom. The rule is also called the duet rule. The only elements known to follow this rule are hydrogen, helium, and lithium. Hydrogen gains and shares electrons as it has only one electron less than the duplet, and lithium loses an electron to attain a duplet. The duplet rule is similar to the octet rule, which states that an atom is stable if it has eight electrons in its valence shell. The octet rule is followed by most elements, except for hydrogen and helium, which are stable in a duplet state instead of an octet state. The octet rule explains why noble gases rarely participate in chemical reactions, as they have reached maximum stability without gaining or losing electrons. The duplet rule explains the stability of helium and the existence of Li⁺¹. The stability of Helium is explained using this rule, and the rule explains the participation of hydrogen in chemical reactions and the stability of hydrogen molecules.