Shannon, R. D., 1976, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides: Acta Crystallographica Section A, v. 32, p. 751–767. DOI: 10.1107/S0567739476001551 provides a very detailed list of ionic radii for just about any element, for various oxidation states and coordination numbers.

This paper was published in 1976. Is there a newer compilation with new data?

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    $\begingroup$ The 82nd edition of the CRC (2001-2002) has tables with data from the Shannon paper you sited plus data from Jia that was published in J. Solid State Chem, 95, 184, 1991. I have not seen the paper, and that is still pretty old, but a bit newer than Shannon anyway. $\endgroup$ – airhuff Jan 31 '17 at 21:55
  • $\begingroup$ @airhuff Of course, the CRC handbook. I'll go and hand a look. $\endgroup$ – Gimelist Jan 31 '17 at 23:52

tldr; it's very difficult to determine consistent additive ionic radii

For various reasons, I've gone back to look at covalent radii recently (e.g., for estimating bond lengths).

In that case, there seem to be fairly useful recent work, e.g. by Pekka Pyykkö: "Additive Covalent Radii for Single-, Double-, and Triple-Bonded Molecules and Tetrahedrally Bonded Crystals: A Summary" J. Phys. Chem. A 2015, 119, 11, 2326–2337

For covalent radii, it's clear that one can fit experimental and theoretical interatomic distance r(A-B) to a sum of two covalent radii with systematic accuracy. Notably, Pyykkö defines not only single-bond but double-bond and triple-bond radii for almost all elements.

(N.B.: I'm not sure it makes sense to consider a triple bond to $\ce{Br}$ or $\ce{I}$ but if you want them, it's in the article.)

There are also recent systematic van der Waals radii, e.g., Cramer and Truhlar: "Consistent van der Waals Radii for the Whole Main Group" J. Phys. Chem. A 2009, 113, 19, 5806–5812

So what about ionic radii?

There's a really nice article by Liu, Schwartz, and Li, cited by Pyykkö about the problems with ionic radii. "On Two Different Objectives of the Concepts of Ionic Radii" Chem (Eur. J.) 2013, 19, 44, 14758-14767

The Liu, Schwartz, and Li point out the competing problems with ionic radii:

Since “bonded atoms” are evidently neither rigid nor spherical, quantum computational chemists apply “valence multiple zeta and polarized” basis sets in their molecular calculations. Conversely, X-ray ED researchers use the procrystal model of independent rigid spherical atoms for starting and screening purposes in their software for the processing and analysis of the observed diffraction data.

In other words, we like to consider atoms as having consistent, rigid spherical shapes. For ionic compounds, that's hard to determine, because of charge transfer. By definition, $\ce{A+B-}$ indicates cation $\ce{A+}$ has given electron density to $\ce{B-}$ .. but how much? Is there any covalent character and shared electron density? How do you divide?

Pyykkö also mentions the problem in his covalent radii work. His atomic radii work:

… provided that the A–B bond is not too ionic, or the coordination numbers of the two atoms too different from the original input data, used in the fit.

In short, covalent radii are easy to define because most of the electron density is shared and covalent bonds are thus fairly predictable. In contrast, almost no compound is purely ionic so it's hard to perform the division for ionic species.

Here are two examples from Liu, Schwartz, and Li:

  1. Consider interatomic distances from alkali halide crystals. (The portion is performed by Bader's AIM analysis.)

anion and cation radii from alkali halide crystals

Note that there's no "vertical" or "horizontal" line for well-defined anion or cation radii. Maybe you can come up with a correction based on electronegativity difference .. but the cation radii particularly depend on the anion.

  1. The same problem shows up for monomeric alkali halide molecules in the gas phase.

enter image description here

It makes some sense. It's difficult to rigorously know how much charge transfers from anion to cation (vs. shared), which is why calculation methods define multiple algorithms to determine it. (Atomic partial charges are something of an eternal debate.)

I wouldn't be surprised to find someone creating a correction using electronegativity differences or covalent character. And it's possible someone will sweep this away with a compelling article - that's the point of science. But it won't be easy, because we expect ionic radii to serve a lot of different purposes.


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