Amount of substance, also called material quantity, is a dimensionless expression of the number of particles in a sample. The particles are usually atom s, but they can be protons, neutrons, electron s, or more fundamental particles such as quarks. Amount of substance can also be expressed in terms of composite particles such as molecules.
The standard unit of material quantity in the International System ( SI ) is the mole . This unit is based on the mass of 0.012 kilogram (kg), or 12 grams (g), of carbon 12 (C-12), the nucleus of which contains six protons and six neutrons. One mole is equal to approximately 6.022169 x 10 23 , and is also called the Avogadro constant.