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