Amount of substance (also called chemical amount) is a quantity defined by standards to measures the size a group of individual elementary entities, such as atoms, molecules and electrons, along with other particles. The International System of Units (SI) defines the amount of substance to be equal to the number of elementary entities present. The SI unit for amount of substance is the mole (mol). The mole is defined as the amount of substance that contains the same number of elementary entities as there are atoms in of the isotope carbon-12. This number is called Avogadro's number and has the value . It is the numerical value of the Avogadro constant which has the unit (or mol), and relates the molar mass of an amount of substance to its mass.