# Synopsis: Chemistry class

#### Determination of the Avogadro Constant by Counting the Atoms in a 28Si Crystal

B. Andreas et al.

Published January 18, 2011

The Avogadro constant—the number of atoms in one mole of an element—provides a link between the atomic and macroscopic properties of matter. One state-of-the-art method for improving the accuracy of this fundamental constant is to use precision x-ray crystallography of highly crystalline silicon spheres: one obtains Avagadro’s number from the ratio of the volume of a mole of silicon (known from its mass) relative to that of a single unit cell in the crystal.

This technique has, however, been plagued by large measurement uncertainties. The main difficulty is accurately determining the isotopic composition of a natural silicon crystal, a key measurement for determining the Avogadro constant. In a paper published in Physical Review Letters, Birk Andreas at Physikalisch-Technische Bundesanstalt in Braunschweig, Germany, with colleagues in Europe and the US report on x-ray studies with a silicon crystal highly enriched with the silicon-$28$ isotope. They compare their results with several others and show a significant improvement in the accuracy of the Avogadro constant, which they determine to be $6.02214078\left(18\right)×{10}^{23}$ with $3.0×{10}^{-8}$ relative uncertainty. Their technique may even allow us to find a replacement for the current platinum-iridium prototype for the value of the kilogram. – Sami Mitra