Also, Rb+, Br-, and Kr are isoelectronic. They have the same shells filled up and the same number of electrons. Rb+ and Br- are VERY close in size with the differences easily explained. It's the Kr atom that just doesn't seem right. The way they tend to determine what an atomic radius is, is to measure the distance between the nucleus of two touching atoms and dividing that by two. I think this might explain the differences here. With a charged ion, that charge is felt over a greater area than just the atom. So it's like trying to put two similar poles of two different magnets together. They just will not want to get close to each other. As a result, their radius will appear to be bigger than it actually is. For a neutral atom, you don't have this repulsion so the radius will be a bit smaller. Hence why the Kr atom has the smaller radius than the Rb+ and Br- atoms. When calculating/measuring the radius of Kr, they don't have to deal with the repulsion between two like charges. When calculating/measuring the radius of an ionic species, you DO have to deal with that repulsion.