The Role ofNAin the SI: An Atomic Path to the Kilogram

The paper summarizes the activities of several national metrological institutes in replacing the kilogram artefact by the mass of a certain number of silicon atoms. This atomic path requires a very precise determination of the Avogadro constant N A with a relative uncertainty of better than 5 × 10 -8 . At present, the most limiting factors are the variability, from sample to sample, of the isotopic abundances and the content of impurities and vacancies, and difficulty in the realization of accurate density standards. Recently, several improvements in molar mass and density determinations have been made, so a further reduction of the level of relative uncertainty towards 10 -7 may be expected in the near future. Beyond that, new experimental methods will be required.