Problems with ionic radii

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 There are several problems in obtaining an accurate set of ionic radii. 

1. Though it is possible to measure the internuclear distances in a crystal very accurately by X-ray diffraction, for example the distance between Na+ and p- in NaF, there is no universally accepted formula for apportioning this to the two ions. Historically several different sets of ionic radii have been estimated. The main ones are by Goldschmidt. Pauling and Ahrens. These are all calculated from observed internuclear distances, but differ in the method used to split the distance between the ions. The most recent values, which are probably the most accurate, are by Shannon (1976) . 

2. Corrections to these radii are necessary if the charge on the ion is changed. 

3. Corrections must also be made for the coordination number, and the geometry. 

4. The assumption that ions are spherical is probably true for ions from the s- and p-blocks with a noble gas configuration, but is probably untrue for transition metal ioris with an incomplete d shell. 

5. In some cases there is extensive delocalization of d electrons, for example in TiO where they give rise to metallic conduction, or in cluster compounds. This also changes the radii. 

Thus ionic radii are not absolute constants, and are best seen as a working approximation.
















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