THE ROLE OF N-A IN THE SI - AN ATOMIC PATH TO THE KILOGRAM

The paper summarizes the activities of several national metrological i nstitutes in replacing the kilogram artefact by the mass of a certain number of silicon atoms. This atomic path requires a very precise dete rmination of the Avogadro constant N-A with a relative uncertainty of better than 5 x 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 realizatio n of accurate density standards. Recently, several improvements in mol ar mass and density determinations have been made, so a further reduct ion of the level of relative uncertainty towards 10(-7) may be expecte d in the near future. Beyond that, new experimental methods will be re quired.