Moles of various elements. All substances are made up of molecules, but the molecules have different mass. Each one of these metals and non-metallic elements has the same number of molecules but a different mass. A mole is 6.0223 x 10 (to the power of 23). A mole of carbon is 12 grams but a mole of mercury is 200.6 grams - the same number of molecules but different masses. A mole is an SI unit defined by the International System of Units.