The easiest way of counting how many particles there are in a substance is to find its mass. There are 6.02×1023 atoms in 12g of carbon-12. It follows that if a magnesium-24 atom is twice the mass of a carbon-12 atom then there are 6.02×1023 atoms in 24g of magnesium-24.The quantity 6.02×1023 is called a mole and the number is called the Avogadro constant and is given the symbol NA.One mole is the number of atoms in 12g of carbon-12.