New Standard Mass

arXiv

Silicon ball picture taken from Nature.

Wikipedia

From the arXiv article:

``The value obtained, N_A = 6.02214084(18) × 10²³ mol⁻¹, is the most accurate input datum for a new definition of the kilogram.''

``The project started in 2004 with the isotope enrichment by centrifugation of SiF₄ gas undertaken at the Central Design Bureau of Machine Building in St. Petersburg. Subsequently, after conversion of the enriched gas into SiH₄, a polycrystal was grown by chemical vapor deposition at the Institute of Chemistry of High- Purity Substances of the Russian Academy of Sciences in Nizhny-Novgorod and, in 2007, the 5 kg ²⁸Si boule shown in Fig. 1 was grown by the Leibniz-Institut f¨ur Kristallz¨uchtung in Berlin [5].''

``Principle of the measurement. Atoms were counted by exploiting their ordered arrangement in the crystal. Provided the crystal and the unit cell volumes are measured and the number of atoms per unit cell is known, the count requires their ratio to be calculated. Hence, $N_A = \frac{nM}{\rho_ 0 a_0^3 }$, where n = 8 is the number of atoms per unit cell, $\frac{M}{\rho_ 0}$ and $a_0^3$ are the molar and unit-cell volumes, M the molar mass and $\rho_0$ the density. The crystal must be free of imperfections, monoisotopic (or the isotopic composition must be determined), and chemically pure. We selected a spherical crystal-shape to trace back the volume determination to diameter measurements and to make possible an accurate geometrical, chemical, and physical characterization of the surface. Hence, two spheres, AVO28-S5 and AVO28-S8, were taken at 229 mm and 367 mm distances, respectively, from the seed crystal position and shaped as quasi-perfect spheres by the Australian Centre for Precision Optics.''