Avogadro's number

Avogadro's number, also called Avogadro's constant (NA) is a large constant used in chemistry and physics. Avogadro's number is formally defined as the number of carbon-12 atoms in 12 grams (0.012 kg) of carbon-12, which is approximately 6.022 × 1023. Historically, carbon-12 was chosen as the reference substance because its atomic mass could be measured particularly accurately.

Additional physical relations

Because of its role as a scaling factor, Avogadro's number provides the link between a number of useful physical constants when we move between an atomic mass scale and a kilogram (SI) scale.

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For example, it provides the relationship between:

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• the universal gas constant R and the Boltzmann constant k: R = k_B N_A
• the Faraday constant F and the elementary charge e: F = e N_A

In the 19th century physicists measured the mass of one atom of hydrogen to be about 1/(6.023x1023) grams. The gram was originally defined to be the mass of a cubic centimeter of pure water at standard temperature and pressure http://www.factmonster.com/ce6/sci/A0832931.html. As experiments became more accurate, it was found that water was contaminated with variable amounts of heavy water, which made it undesirable to maintain a standard with hydrogen having one a.m.u. (atomic mass unit). Carbon was found to have a more constant isotopic composition, and it was also possible to separate pure carbon-12. Therefore, the atomic mass unit was changed to 1/12 the mass of an atom of carbon-12 . Hence 12 grams of carbon-12 has about 6.02214x1023 atoms. The recent history and more details can be found in the document, Atomic Weight: The Name, Its History, Definition and Units.

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