I was wondering why the potassium ion (K+) is larger than the strontium ion (Sr2+). From my understanding, the addition of the n=4 electron shell should result in the strontium ion being larger. However, that is not the case. I think the difference has something to do with the additional two protons in strontium 2+ relative to it's nearest noble gas Kr (K+ only has one additional proton commpared to Ar).
However, without searching up the atomic radii (or anything outside of what's in the periodic table), how can I compare the radii of two ions with different charges like K+ and Sr2+?