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One mole of carbon. Carbon powder being weighed out on a balance. 12 grams of carbon is 1 mole. 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 vital concept in understanding chemical reactions and calculations. The mole was defined in 1971 in relation to the number of atoms in 12 grams of the most abundant isotope of carbon. This immensely large number (Avogadro's number) is over 600 billion trillion atoms, or a six followed by 23 zeros. All atomic weights are given relative to carbon's atomic weight of 12 grams per mole. (KEYSTONE/SCIENCE PHOTO LIBRARY/ANDREW LAMBERT PHOTOGRAPHY)