Physikalisch-Technische Bundesanstalt

The mole 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.

Actually, the mole is nothing else but a periphrasis for a large number: It indicates the number of particles contained in the mass of a substance, which corresponds to its atomic mass in g – in the case of carbon (with the atomic mass 12 u) the number of particles contained in 12 g. This number is always the same – about 6.022 · 10²³, a numerical value which is referred to as the Avogadro constant (N_A). Well then, what is 1 mol? An amount consisting of N_A particles. It is a unit created for chemistry to allow the interaction of particles in reactions to be determined.

The realization of the mole thus requires that particles be counted. In the PTB's Avogadro project the particles in a silicon crystal are concerned. It is the aim of this project to fabricate such a pure crystal or to develop such good counting methods that extrapolation from a small subset is sufficient: count the particles in a part of the crystal and calculate from them the number of particles contained in one mole of silicon.

Some problems still remain to be solved; for example, the problem that foreign atoms, or no atoms at all, are located in certain sections of such a crystal. Once this task is fulfilled, the researchers will have killed two birds with one stone: They will have developed, first, a very precise method for the realization of the mole and, secondly, a new method for the realization of the SI base unit, the kilogram, on the basis of the fundamental constant N_A. Both units are so far realized by comparisons. A weight of unknown mass is compared with the prototype kilogram. For the determination of the molar mass of a substance, it is compared with the molar mass of carbon (¹²C), which can be determined with high accuracy by means of mass spectrometers.