ATOMIC WEIGHTS AND FUNDAMENTAL CONSTANTS

A consistent set of internationally accepted atomic weights has long b een the aim of the scientific community, because of the relevance of t hese values to science and technology, as well as to trade and commerc e. Atomic weights were originally determined by chemical stoichiometry , but are now based almost entirely on a knowledge of the isotopic com position and atomic masses of an element. Technological advances in ma ss spectrometry have not only enabled atomic masses to be determined w ith an accuracy better than one part in 10(7), and the absolute isotop e abundances of an increasing number of elements to be measured, but h ave also led to the realisation that many elements display small but s ignificant variations in their isotope abundances, caused by a variety of physicochemical processes in nature. Although such variations plac e a constraint on the certainty with which some atomic weights can be evaluated, these variations provide the opportunity of investigating a range of important geochemical problems. Accurate determinations of t he atomic weights of certain elements also affect the values of a numb er of fundamental constants such as Avogadro's constant, the Faraday c onstant and the universal gas constant, and enable the uncertainties a ssociated with their values to be reduced. This in turn influences the set of selfconsistent values of other basic constants through a least squares adjustment methodology. This review describes the role of the International Union of Pure and Applied Chemistry's Commission on Ato mic Weights and Isotopic Abundances in providing accurate information on the isotopic compositions and atomic weights of the elements to the scientific community, and their impact on fundamental constants.