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Physics Today

Negative Calcium Ions Are Stable—In Spite of the Conventional Wisdom

Bertram M. Schwarzschild

Published Jun 1, 1988• 4 citations
Publisher

Abstract

Most atomic species—roughly 80% of the periodic table—can form stable negative ions in their gaseous phase. The most notable and easily understood exceptions are the noble gas atoms, the symmetry of whose completely filled electron shells offers no handhold for a would-be extra tenant. The alkaline earths—calcium, strontium, barium and their group IIA cousins—are similarly inhospitable, or so the standard textbooks assure us. Two back-to-back reports in a recent issue of Physical Review Letters have now disabused us of this venerable assurance.

DOI
10.1063/1.2811444

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Most atomic species—roughly 80% of the periodic table—can form stable negative ions in their gaseous phase. The most notable and easily understood exceptions are the noble gas atoms, the symmetry of whose completely filled electron shells offers no handhold for a would-be extra tenant. The alkaline earths—calcium, strontium, barium and their group IIA cousins—are similarly inhospitable, or so the standard textbooks assure us. Two back-to-back reports in a recent issue of Physical Review Letters have now disabused us of this venerable assurance.
Strontium, barium and radium are predicted to have stable negative ions of ns2np designation, on the basis of eigenchannel R-matrix calculations performed in LS coupling for the negative ion of each alkaline-earth atom. The calculations also confirm earlier studies indicating that the negative ion of calcium is stable and that corresponding 2P degrees states in beryllium and magnesium are unstable shape resonances. The much larger polarisability of the heavier alkaline earths play a crucial role in binding the additional electron. The analysis uses quantum defect theory in its generalised form appropriate for long-range motion in a dipole polarisation potential.
A chilly meeting of barium and calcium The periodic table is an excellent predictor of element ratios in chemical compounds that form at temperatures that we commonly experience, give or take a factor of 10. Strange things start happening at hot and cold extremes, though. Puri et al. take advantage of extreme cold to observe the formation of the BaOCa + ion, an electron-deficient alternative to conventional binary barium or calcium oxides. They first prepared cold barium methoxide ions and then exposed them to calcium atoms cooled to thousandths of a kelvin. Mass spectral and theoretical analyses revealed a barrierless reaction pathway in which triplet-state calcium displaces the methyl group. Science , this issue p. 1370