The ionic radius is not a fixed property of a given ion, but varies with [[coordination number]], [[spin states (d electrons)|spin state]] and other parameters. Nevertheless, ionic radius values are sufficiently [[Transferability|transferable]] to allow [[Periodicity|periodic trends]] to be recognized. As with other types of [[atomic radius]], ionic radii increase on descending a [[Periodic table group|group]]. Ionic size (for the same ion) also increases with increasing coordination number, and an ion in a [[spin states (d electrons)|high-spin]] state will be larger than the same ion in a [[low-spin]] state. Anions (negatively charged) are almost invariable larger than cations (positively charged), although the [[fluoride]]s of some [[alkali metal]]s are rare exceptions. In general, ionic radius decreases with increasing positive charge and increases with increasing negative charge.
Unit cell parameters (in pm, equal to two M–X bond lengths) for sodium and silver halides. All compounds crystallize in the NaCl structure.
An "anomalous" ionic radius in a crystal is often a sign of significant covalent character in the bonding. No bond is completely ionic, and some supposedly "ionic" compounds, especially of the transition metals, are particularly covalent in character. This is illustrated by the unit cell parameters for sodium and silverhalides in the table. On the basis of the fluorides, one would say that Ag+ is larger than Na+, but on the basis of the chlorides and bromides the opposite appears to be true.[3] This is because the greater covalent character of the bonds in AgCl and AgBr reduces the bond length and hence the apparent ionic radius of Ag+, an effect which is not present in the halides of the more electropositive sodium, nor in silver fluoride in which the fluoride ion is relatively unpolarizable.
Ionic radii 6 coordinate unless marked (e.g +34). ls = low spin, hs= high spin. [2]
