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
International System of Units Another three base units (for temperature, amount of substance and luminous intensity) were added later. The early metric systems defined a unit of weight as a base unit, while the SI defines an analogous unit of mass. In everyday use, these are mostly interchangeable, but in scientific contexts the difference matters. Mass, strictly the inertial mass, represents a quantity of matter. It relates the acceleration of a body to the applied force via Newton's law, : force equals mass times acceleration. A force of 1 N (newton) applied to a mass of 1 kg will accelerate it at 1 m/s.