- (mol)the SI base unit of the amount of a substance (as distinct from its mass or weight).Moles measure the actual number of atoms or molecules in an object. An earlier name is gram molecular weight, because one mole of a chemical compound is the same number of grams as the molecular weight of a molecule of that compound measured in atomic mass units. The official definition, adopted as part of the SI system in 1971, is that one mole of a substance contains just as many elementary entities (atoms, molecules, ions, or other kinds of particles) as there are atoms in 12 grams of carbon-12 (carbon-12 is the most common atomic form of carbon, consisting of atoms having 6 protons and 6 neutrons). The actual number of "elementary entities" in a mole is called Avogadro's number after the Italian chemist and physicist Amedeo Avogadro (1776-1856). Careful measurement determines Avogadro's number to be approximately 602.214 179 x 1021. In the American system of naming big numbers, that's 602 sextillion 214 quintillion 179 quadrillion, give or take about 50 quadrillion.
Dictionary of units of measurement. 2015.