The relative atomic mass of an element is the average mass of its isotopes compared with an atom of carbon-12. Isotopes occur when atoms have different numbers of neutrons but the same number of protons. A mole is a specific amount of atoms. One mole equals 6.02 x 1023 atoms. This number is known as Avogadro's number. A mole of a substance is obtained by weighing out the RAM or formula mass in grams. This is because, for example, 12 grams of carbon contains 6.02 x 1023 atoms. 18 grams of water would contain 6.02 x 1023 molecules of water, i. e. 1 mole. The formula of a compound tells you how many atoms of each element combine. The empirical formula is the simplest ratio of atoms in a compound.