Silicon

The standard kilogram bar,which is an alloy of platinum and iridium, has been held in the vaults of the International Bureau of Weights and Measures in Paris since 1889, but has proved not to be quite so permanent as assumed and has lost 50 micrograms over the years. The alternative standard weight is likely to become the perfect sphere of silicon that has been made entirely of the isotope silicon-28 (which constitutes 92 per cent of this element) and is the work of scientists in Russia, Germany, and Australia. Knowing the exact number of atoms it contains will enable its exact weight to be calculated.