A unit (symbolized mol) used to measure the amount of material in a chemical sample. Gram-molecular weight is an older name for the mole and is seldom used. The mole is defined by international agreement as the amount of substance (chemical amount) of a chemical system that contains as many molecules or entities as there are atoms in 12 g of carbon-12 (¹²C). When the mole is used, the elementary entities need not be molecules, but they must always be specified. They may be atoms, molecules, ions, electrons, or specified groups of such particles.