Atoms, molecules and ions are far too small to count one at a time, so chemists count them in large batches called moles. One mole of a substance is the amount that contains the Avogadro constant number of particles. The Avogadro constant is 6.02 × 1023. The particles can be atoms, molecules, formulae or ions, depending on the substance.
The Avogadro constant is the same for every substance. One mole of carbon contains 6.02 × 1023 atoms, and one mole of water contains 6.02 × 1023 molecules. Because the particles of different substances have different masses, equal numbers of them have different masses.
A mole is also defined by mass. The mass of one mole of a substance, in grams, is equal to its relative particle mass. The relative particle mass is the relative atomic mass for an element, or the relative formula mass for a compound. So one mole of carbon, with a relative atomic mass of 12, has a mass of 12 g. One mole of water, with a relative formula mass of 18, has a mass of 18 g. One mole of sodium chloride, with a relative formula mass of 58.5, has a mass of 58.5 g.