Counting a mole of silicon atoms

Counting a mole of silicon atoms. Avogadro’s number, N _A, links the microscopic and macroscopic worlds by specifying how many individuals make up a mole. Now an international team of metrologists has obtained N _A with an unprecedented precision of 30 parts per billion. The result: N _A = 6.02214078(18) × 10²³. The idea behind the new experiment is simple. A sample from a crystalline silicon-28 boule, shown in the figure, was subjected to x-ray interferometry, which yielded the volume of the 8-atom Si unit cell. Other bits of the boule were painstakingly fabricated into spheres whose volumes and masses were carefully measured. The spherical volume divided by the unit-cell volume gives the number of Si atoms; the mass gives the number of moles. Voilà, atoms per mole. The devil, of course, is in the details. The team needed to measure and account for such flaws...