Learn to convert between moles, grams, atoms and molecules. Do not forget to subscribe. Click to see more information A mole is defined as the amount of substance containing 6.023 x 10 ^ 23 particles, whether of an element or a compound. In an element this amount is equivalent to the atomic mass expressed as grams. For example, in 15.99 grams of oxygen there are exactly 6.02 x 10 ^ 23 oxygen atoms. This number is known as Avogadro number, because it was the Italian chemist Amadeo Avogadro (1776-1856) who established this rule. Avogadro discovered that equal volumes of different gases, under the same conditions of temperature and pressure, contained equal numbers of molecules. If a number of atoms of two different elements have different masses, we can establish how heavy one is in relation to the other. Thus, if one mole of oxygen weighs 16 g, while one mole of carbon weighs 12 g, we can easily conclude that the oxygen atoms are heavier than the carbon atoms. The Avogadro number is a very important concept and very useful in chemistry. For example, it serves to calculate the relative mass of an atom of any element and the number of atoms or particles present in a given mass of a given substance. 1 mol contains 6.02 x 10 ^ 23 particles, atoms or molecules whose mass is equal to the mass of the element or compound.