What is a mole?

A mole is the amount of a substance which contains as many particles (atoms, ions, molecules etc) of the substance as there are carbon atoms in 12g of ¹²C - this is exactly 6.02 × 10²³ atoms. This formula is known as the Avogrado number. The mass in grams of any element is equal to its relative atomic mass - for example, one mole of Calcium is approximately 40g.