W0071

Atomic-Weight Determinations from Crystal Data; Sparse History; any Future Prospects? Richard D. Deslattes, National Institute of Standards and Technology, Gaithersburg MD 20899 and H. Steffen Peiser, Asbury Retirement Community, Gaithersburg MD 20877

In recent years, direct determinations of the Avogadro constant, N_A -- the factor that relates atomic-scale to macroscopic unit quantities -- have been based on measurements of the unit-cell volume of monocrystalline silicon, its macroscopic density, and an independent Si atomic-weight (mean relative atomic mass) determination. N_A can also be obtained with comparable uncertainty from least-squares analysis of other measurements of fundamental constants. Given a value of N_A from either or both of these sources, one can derive mean molar-mass values (numerically equal to atomic weights) of other substances. These follow from the mass content of a unit cell, as obtained from a substance's density and unit-cell volume, provided the substance can be realized as a well characterized single crystal. For crystals of a pure element, and in the case of a simple stoichiometric compound involving, for example, mononuclidic chemical partners, the average atomic weight emerges directly without requiring­isotopic-abundance measurements.

In the history of the IUPAC Standard Atomic Weights Table, which reflect the ranges of isotopic abundances in normal terrestrial sources, only the evaluated values for silicon, calcium, and germanium have so far been influenced by crystal data. The variability of the isotope abundances is often larger, especially for low atomic-number elements, than available measurement precision in mass spectrometry, in the x-ray/crystal density (XRCD) approach, or even from analytical chemical methods.

It is, however, precisely this variability which is of interest in many geological, archaeological, and biological investigations. Since most materials can be dissolved in appropriate solvents, purified, and reacted to form crystallizable salts, there is a reasonably general opportunity to apply the XRCD approach. An otherwise formidable physical problem is thereby solved with several advantages including rapid processing of large numbers of samples at low cost with modest resources in technologically limited environments. Among the elements for which such an approach may be appropriate, despite its ultimately limited accuracy, are those having many isotopes, such as titanium, germanium, tin, and rare-earth elements with even atomic number. Similar considerations may commend the XRCD approach to such elements as boron, nitrogen, sulfur, and chlorine with significant applications also in studies of technological processes.

Key words: Atomic weights; Avogadro constant; Unit cell density.