Ionic Radii

ionic radii [ī′än·ik ′rād·ē‚ī]

(physical chemistry)

Radii which can be assigned to ions because the rapid variation of their repulsive interaction with distance makes them repel like hard spheres; these radii determine the dimensions of ionic crystals.

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Ionic Radii 

arbitrary characteristics of ions used for an approximate estimation of the internuclear distances in ionic crystals. The values of ionic radii are related in a regular fashion to the positions of the elements in Mendeleev’s periodic system. Ionic radii are widely used in crystal chemistry, making it possible to reveal the structural relationships in crystals of various compounds, and in geochemistry, for studies of ion substitution phenomena in geochemical processes.

Several systems of values of ionic radii have been proposed. As a rule, they are based on the following observation: the difference in the internuclear distances A-X and B-X in ionic crystals of composition AX and BX, where A and B are metals and X is a nonmetal, is virtually constant when an analogous nonmetal is substituted for X (for example, substituting bromine for chlorine), provided that the coordination numbers of the analogous ions are identical in the salts being compared. From this it follows that ionic radii have the property of additivity—that is, the experimentally determined internuclear distances may be regarded as the sum of the corresponding ionic “radii.” The breakdown of the sum into components is always based on more or less arbitrary assumptions. The systems of ionic radii proposed by various authors differ mainly in the use of various initial assumptions.

Ionic radii are tabulated for various values of the oxidation number. When its value is different from +1, the oxidation number does not correspond to a real degree of ionization of atoms, and the corresponding ionic radii acquire an even more arbitrary meaning, since the bond may be covalent to a considerable degree. The values of some ionic radii (in angstroms) for some elements (according to N. V. Belov and G. B. Bokii) are as follows: F^-, 1.33; Cl^-, 1.81; Br,^- 1.96; I^-, 2.20; O²1.36; Li⁺, 0.68; Na⁺, 0.98; K⁺, 1.33; Rb⁺, 1.49; Cs⁺, 1.65; Be²⁺, 0.34; Mg²⁺, 0.74;Ca²⁺, 1.04; Sr²⁺, 1.20; Ba²⁺, 1.38; Sc³⁺, 0.83; Y³⁺, 0.97; La³⁺, 1.04.

V. A. KIREEV