Okay I think this question has been addressed slightly before however I seem to be having difficulty on it still. The question asks: Arrange the following atoms or ions in order of increasing radius Cl, S2-, K, K+, O Now I have them arranged as O < Cl < K+ < S2- < K The arrangement of K+ and S2- is where I am stumped. The question goes on to ask "Give an explanation for the position of S2- in relation to the atom or ion that comes just before and just after" Now I would only assume they're talking about the K+ and K atom/ion so I placed S2- in between. Now my best guess at why S2- is where it is, is due to the gain in electrons. But how do I prove this? Is it because the S atom has gained two electrons and the K atom has only lost one that the S2- ion becomes larger than the K+ ion? Is there a certain method I should be using to determine the actual resultant size of these atoms when they gain or lose electrons? I tried to keep the post as informative as possible I'm not trying to scam any answer out of anyone I'm trying to learn how to solve this problem.