isotope syntax: An isotope in the ScientificConstants package is specified by using one of the three forms of the isotope syntax: an&comma;mn, Xmn, yyyymn, Xmn, or yyyymn, where an is the atomic number, mn is the mass number, X is the element symbol, and yyyy is the element name. It is an error to specify a mass number less than the (given or implied) atomic number.