Atomic Weights and Fundamental Constants

Abstract

A consistent set of internationally accepted atomic weights has long been the aim of the scientific community, because of the relevance of these values to science and technology, as well as to trade and commerce. Atomic weights were originally determined by chemical stoichiometry, but are now based almost entirely on a knowledge of the isotopic composition and atomic masses of an element. Technological advances in mass spectrometry have not only enabled atomic masses to be determined with an accuracy better than one part in 10⁷, and the absolute isotope abundances of an increasing number of elements to be measured, but have also led to the realisation that many elements display small but significant variations in their isotope abundances, caused by a variety of physicochemical processes in nature. Although such variations place 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 the atomic weights of certain elements also affect the values of a number of fundamental constants such as Avogadro's constant, the Faraday constant and the universal gas constant, and enable the uncertainties associated 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 Atomic 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.