# Synopsis: Chemistry class

Precision x-ray measurements hone in on a more accurate value of Avogadro’s constant.

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

### Announcements

More Announcements »

## Subject Areas

Atomic and Molecular Physics

Optics

Soft Matter

## Related Articles

Atomic and Molecular Physics

### Synopsis: Rapid Alignment

A frequency comb can align an ensemble of molecules 150 million times per second. Read More »

Atomic and Molecular Physics

### Viewpoint: Negative Ions in Cold Storage

A cooled ring stores high-speed negative ions for more than 1000 seconds and enables new studies of atomic and molecular ions that are important in interstellar and atmospheric chemistry. Read More »

Atomic and Molecular Physics

### Synopsis: Spin-Orbit-Coupled Photons

Photons confined to a hexagonally shaped microcavity move in a polarization-dependent way, thus simulating a spin-orbit coupling common in materials. Read More »