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.

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

References in periodicals archive ?
There is a set of divalent and trivalent cations what can be combined to form stable hydrotalcite structures due limitations in oxidation states and ionic radii. The hexacoordinated Mg(II) have an ionic radius of 0.072 nm so that the hexacoordinated Al(III), with ionic radius of 0.054 nm [19] cause a slight lattice distortion in the lamella, originating a smoothed zigzag in surface.
However, it is still necessary to consider effective ionic radii much larger than their hydrated values in order to fit experimental data.
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Because the ionic radii of Sr2+, Yb3+ and In3+ are larger than those of Ca2+ and Fe3+, there is lower activation energy required for oxygen ion conductivity than CaFe2O4 type materials.
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This infers the crystalinity of ZnO particles might be distorted by the introduction of dopant and whose ionic radii are lesser than that of Zn.
The lattice parameters a-axis and c-axis of Ni doped ZnS samples decrease with the increase of [Ni.sup.2+] doped content, which can be attributed to the substitution of smaller ionic radii [Ni.sup.2+] (0.68 [Angstrom]) into larger ionic radii [Zn.sup.2+] (0.74 [Angstrom]) in the wurtzite structure and causing lattice distortion.
Although [Na.sup.+] and [K.sup.+] are both monovalent cations, and [Na.sup.+] and [Ca.sup.2+] have similar ionic radii, their behavior under hydration is different [17].
The variation can be explained on the basis of ionic radii of the substituted ions.
Dopant owns physical properties like higher ionic radii mobility and preference occupancy were the reasons for generation impurity phases.
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