H-rule (hydrogen and halogen rule; hydrogen rule): For a molecule containing only hydrogen, carbon, oxygen, nitrogen, fluorine, chlorine, bromine, and iodine, the maximum number of monovalent atoms possible (max H) for a given number of carbons (C) and nitrogens (N) is given by the equation max H = 2C + N + 2. Note this gives the maximum number; less may be present. The rule may or may not apply if other elements such as sulfur or phosphorous are present. Often called the Hydrogen Rule, but this is misleading because the rule gives the maximum number of hydrogens plus halogens.