Avogadro constant

The Avogadro constant (symbol: L or N_A) is a physical constant that relates measurements made at the macroscopic scale to those made on an atomic scale. It is usually defined as the number of entities (such as atoms or molecules) in one mole, and the 2006 CODATA recommended value is 6.022 141 79(30) × 10²³ mol⁻¹. It was orginally called Avogadro's number, and is named in honour of the Italian physicist Amadeo Avogadro.

International Avogadro Coordination

The International Avogadro Coordination (IAC), often simply called the "Avogadro project", is a collaboration begun in the early 1990s between various national metrology institutes to measure the Avogadro constant by the X-ray crystal density method to a relative uncertainty of 2 × 10⁻⁸ or less. The project is part of the efforts to redefine the kilogram in terms of a universal physical constant, rather than the International Prototype Kilogram, and compliments the measurements of the Planck constant using watt balances. Under the current definitions of the International System of Units (SI), a measurement of the Avogadro constant is an indirect measurement of the Planck constant:

<math>h = \frac{c\alpha^2 A_{\rm r}({\rm e})M_{\rm u}}{2R_{\infty}} N_{\rm A}</math>

The measurements use highly polished spheres of silicon with a mass of one kilogram. Spheres are used to simplify the measurement of the size (and hence the density) and to minimize the effect of the oxide coating that inevitably forms on the surface. The first measurements used spheres of silicon with natural isotopic composition, and had a relative uncertainty of 3.1 × 10⁻⁷. These first results were also inconsistent with values of the Planck constant derived from watt balance measurements, although the source of the discrepancy is know believed to be known.

The main residual uncertainty in the early measurements was in the measurement of the isotopic composition of the silicon to calculate the atomic weight so, in 2007, a 4.8-kg single crystal of isotopically-enriched silicon (99.94% ²⁸Si) was grown, and two one-kilogram spheres cut from it. Diameter measurements on the spheres are repeatable to within 0.3 nm, and the uncertainty in the mass is 3 µg. Full results from these determinations are expected in late 2010.

Proposed redefinition

In the context of the "New SI", based around the redefinition of the kilogram in terms of a universal physical constant, there have been several proposals that would give the Avogadro constant a defined value in SI units.