# Gap in ionic radii

The above picture was shown in a presentation by my prof in inorganic chem as we were discussing ionic radii more in depth (all in pm). These are all isoelectronic radii. Notice the sudden leap in radii going from the 6th main group to the 5th main group. Its about 30 pm overall. Now how can that be? Is this a known thing?

You lose a proton in the nucleus, but so do you when you go from the halides to the 6th main group.

I unfortunately don't know the source of this picture, but my prof was sure that it came from a reliable source.

• 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 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 at 21:23
• This might be helpful, particularly table 8.2.3, data from R. D. Shannon, “Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides,” Acta Crystallographica 32, no. 5 (1976): 751–767. – Buck Thorn Mar 20 at 21:39
• @NightWriter yeah there seems to be newer evidence that doesnt show such a big gap...but I have to look a little deeper before making a final statement – TheChemist Mar 22 at 21:09
• @MathewMahindaratne well its not that surprising since group VI and VII have very similar radii without any charge if you look at the table Night Writer provided – TheChemist Mar 22 at 21:10