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.

Numerical value

At present it is not technologically feasible to count the exact number of atoms in .012 kg of carbon-12, so the precise value of Avogadro's number is unknown. The 2002 CODATA recommended value for Avogadro's number is

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:6.0221415(10) imes 10^{23}hbox{ mol}^{-1},

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where the number in parentheses represents the one standard deviation uncertainty in the last digits of the value.

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A number of methods can be used to measure Avogadro's number. One modern method is to calculate Avogadro's number from the density of a crystal, the relative atomic mass, and the unit cell length determined from x-ray crystallography. Very accurate values of these quantities for silicon have been measured at the National Institute of Standards and Technology (NIST) and used to obtain the value of Avogadro's number.

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