This can be done, however, and chemists use a certain standard number of particles as a basis for comparing different substances. This standard number is contained in an amount of substance known as a mole, the most common number of atoms referred to by chemists. This number of formula units in a mole is called Avogadro number, usually symbolised by N 0 . This is a fundamental quantitative relationship between microscopic and macroscopic chemistry. Before 1909 rough values of N 0 had been estimated in various ways.
Some methods depend directly or indirectly on the kinetic theory of gases.
Reference has been made to the determination of this number by Lord Rayleigh through the consideration of the blue colour of the sky. X-ray crystallographic method of determining N 0 is one of the most accurate methods now known. Similarly the relative molecular mass of carbon dioxide is 44.