- Ionic radius
The ionic radius, "r"ion, is a measure of the size of an
ionin a crystal lattice. It is measured in either picometres (pm) or Angstrom (Å), with 1 Å = 100 pm. Typical values range from 30 pm (0.3 Å) to over 200 pm (2 Å).
The concept of ionic radius was developed independently by Goldschmidt and Pauling in the 1920s to summarize the data being generated by the (then) new technique of
X-ray crystallography: it is Pauling's approach which proved to be the more influential. X-ray crystallography can readily give the length of the side of the unit cellof a crystal, but it is much more difficult (in most cases impossible, even with more modern techniques) to distinguish a boundary between two ions. For example, it can be readily determined that each side of the unit cell of sodium chlorideis 564.02 pm in length, and that this length is twice the distance between the centre of a sodium ion and the centre of a chlorideion:::2 ["r"ion(Na+) + "r"ion(Cl−)] = 564.02 pmHowever, it is not apparent what proportion of this distance is due to the size of the sodium ion and what proportion is due to the size of the chloride ion. By comparing many different compounds, and with a certain amount of chemical intuition, Pauling decided to assign a radius of 140 pm to the oxideion O2−, at which point he was able to calculate the radii of the other ions by subtraction.Pauling, L. (1960). " The Nature of the Chemical Bond" (3rd Edn.). Ithaca, NY: Cornell University Press.]
A major review of crystallographic data led to the publication of a revised set of ionic radii in 1976," Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides" Shannon R.D. Acta Cryst. A32 751-767 (1976) doi|10.1107/S0567739476001551] and these are preferred to Pauling's original values. Some sources have retained Pauling's reference of "r"ion(O2−) = 140 pm, while other sources prefer to list "effective" ionic radii based on "r"ion(O2−) = 126 pm. The latter values are thought to be a more accurate approximation to the "true" relative sizes of
anions and cations in ionic crystals.
The ionic radius is not a fixed property of a given ion, but varies with
coordination number, spin stateand other parameters. Nevertheless, ionic radius values are sufficiently transferable to allow periodic trends to be recognized. As with other types of atomic radius, ionic radii increase on descending a group. Ionic size (for the same ion) also increases with increasing coordination number, and an ion in a high-spinstate will be larger than the same ion in a low-spinstate. Anions (negatively charged) are almost invariable larger than cations (positively charged), although the fluorides of some alkali metals are rare exceptions. In general, ionic radius decreases with increasing positive charge and increases with increasing negative charge.
* [http://www.webelements.com/ WebElements]
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