Moles of sodium chloride and water. 200 grams of water (centre right) has the same number of molecules as 58.5 grams of sodium chloride (lower left). A mole is a chemical unit of measurement that shows the number of molecules or atoms present. One mole of any substance has the same number of chemical units, a concept used in relation to chemical reactions and calculations. The mole, one of the base international (SI) units, was defined in 1971 in relation to the number of atoms in 12 grams of the most abundant isotope of carbon. Known as Avogadro's number, this immensely large quantity is over 600 billion trillion atoms, or a six followed by 23 zeros.