Atomic radius

Abstract: A new set of covalent atomic radii has been deduced from crystallographic data for most of the elements with atomic numbers up to 96. The proposed radii show a well behaved periodic dependence that allows us to interpolate a few radii for elements for which structural data is lacking, notably the noble gases. The proposed set of radii therefore fills most of the gaps and solves some inconsistencies in currently used covalent radii. The transition metal and lanthanide contractions as well as the differences in covalent atomic radii between low spin and high spin configurations in transition metals are illustrated by the proposed radii set.

Abstract: We report here an approach for predicting charge distributions in molecules for use in molecular dynamics simulations. The input data are experimental atomic ionization potentials, electron affinities, and atomic radii. An atomic chemical potential is constructed by using these quantities plus shielded electrostatic interactions between all charges. Requiring equal chemical potentials leads to equilibrium charges that depend upon geometry. This charge equilibration (QEq) approach leads to charges in excellent agreement with experimental dipole moments and with the atomic charges obtained from the electrostatic potentials of accurate ab initio calculations. QEq can be used to predict charges for any polymer, ceramic, semiconductor, or biological system, allowing extension of molecular dynamics studies to broad classes of new systems. The charges depend upon environment and change during molecular dynamics calculations. We indicate how this approach can also be used to predict infrared intensities, dielectric constants, and other charge-related properties.

Abstract: A set of empirical atomic radii has been set up, such that the sum of the radii of two atoms forming a bond in a crystal or molecule gives an approximate value of the internuclear distance. These radii give fair agreement with experiment in over 1200 cases of bonds in all types of crystals and molecules, with an average deviation of about 0.12 A. The radii are similar to a set suggested by W. L. Bragg in 1920, but refined by consideration of many more crystals. They hold for covalent, metallic, and ionic binding equally well. These radii agree remarkably well with calculated radii of maximum radial charge density in the outermost shells of the atoms, as taken from previously unpublished calculations of D. Liberman, J. T. Waber, and D. T. Cromer, of the Los Alamos Laboratory, by relativistic self‐consistent field calculation, using the exchange correction suggested in 1951 by the present author. There is discussion of the probable reason for this agreement, and discussion of the relation of these radii to the ionic radii of Pauling, Zachariasen, and others, and the tetrahedral and metallic radii of Pauling.

Abstract: Unlike the well-defined long-range order that characterizes crystalline metals, the atomic arrangements in amorphous alloys remain mysterious at present. Despite intense research activity on metallic glasses and relentless pursuit of their structural description, the details of how the atoms are packed in amorphous metals are generally far less understood than for the case of network-forming glasses. Here we use a combination of state-of-the-art experimental and computational techniques to resolve the atomic-level structure of amorphous alloys. By analysing a range of model binary systems that involve different chemistry and atomic size ratios, we elucidate the different types of short-range order as well as the nature of the medium-range order. Our findings provide a reality check for the atomic structural models proposed over the years, and have implications for understanding the nature, forming ability and properties of metallic glasses.