Avogadro constant of molecules in one gram-molecule of oxygen (exactly of oxygen, according to the definitions of the period), and this term is still widely used, especially in introductory works. The change in name to "Avogadro constant" ("N") came with the introduction of the mole as a base unit in the International System of Units (SI) in 1971, which regarded amount of substance as an independent dimension of measurement. With this recognition, the Avogadro constant was no longer a pure number, but had a unit of measurement, the reciprocal mole (mol). While it is rare to use units of amount of substance
Internal energy density called "specific internal energy" which is internal energy per unit of mass (kilogram) of the system in question. The SI unit of specific internal energy is J/kg. If the specific internal energy is expressed relative to units of amount of substance (mol), then it is referred to as "molar internal energy" and the unit is J/mol. From the standpoint of statistical mechanics, the internal energy is equal to the ensemble average of the sum of the microscopic kinetic and potential energies of the system. The internal energy, , expresses the thermodynamics of a system in the "energy-language", or in