Mole
The mole (mol) is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles or specified groups of such particles. In this definition, it is understood that unbound atoms of carbon 12, at rest and in their ground state, are referred to.
The amount of a substance can be specified in principle by simply counting its constituents (atoms, molecules, etc.) one by one. For that, no new unit is necessary at all. The problem is that atoms and molecules are so tiny and their number in macroscopic amounts is so large, that normal counting is not possible.
However, when the ratios of atomic or molecular masses are known, the amount of a substance can be determined as well by weighing. The reference measure for weighing is the mole. In a mole of a pure substance there are as many entities as there are atoms in 0.012 kilogram of carbon 12. This number is called Avogadro constant - formerly also known as Loschmidt's number - and its value amounts to (6.022 1415 ± 0.000 001 0) · 1023 mol-1. The mole was adopted as seventh base unit of the SI by the 14th General Conference on Weights and Measures in 1971.