I'm teaching periodic trends in a Gen. Chem. course, and it's commonly mentioned that the radius of an Li+ ion (90 pm) is smaller than that of the neutral Li atom (~160 pm), with the dominant reason being that the valence 2s1 electron was removed. The same trend shows up for the other alkali metals, naturally. However, the ionic radius of Li+ is 90 pm, while the atomic radius of He is 31 pm (calculated). The strange thing (to me, anyway) is that they both have the same electron configuration, namely two electrons in the 1s orbital. Li+ has the larger nuclear charge, so I'd have expected Coulomb's law to pull those two electrons closer to the nucleus, but apparently the exact opposite result happens. Anyone know what I'm missing? Similar trends seem to be observed for the other pairs of alkali ions/noble gases... Thanks!