• When you go from He to Be+2, Ne to Mg+2 and so on, then you're losing the whole s shell from the next $n$ quantum number, but adding two protons. Why shouldn't the ion be much smaller? – MaxW Mar 19 '19 at 21:08
• Actually for me, the surprise is difference between main groups VI and VII. For example, $\ce{S^2-}$ to $\ce{Cl-}$ is only $\pu{3 pm}$ while $\ce{O^2-}$ to $\ce{F-}$ is $\pu{4 pm}$. On the other hand, others have more than $\pu{20 pm}$ difference. – Mathew Mahindaratne Mar 19 '19 at 21:23