A pair of oppositely charged ions held together by @C01365@ attraction without formation of a @C01384@. Experimentally, an ion pair behaves as one unit in determining conductivity, kinetic behaviour, osmotic properties, etc. Following Bjerrum, oppositely charged ions with their centres closer together than a distance: \[q = \frac{8.36\times 10^{6}\ z^{+}\ z^{-}}{\varepsilon_{\text{r}}\ T}\ \text{pm}\] are considered to constitute an ion pair ('Bjerrum ion pair'). [\(z^{+}\) and \(z^{-}\) are the charge numbers of the ions, and \(\varepsilon_{\text{r}}\) is the @R05273@ (or @D01697@) of the medium.] An ion pair, the @C01281@ ions of which are in direct contact (and not separated by an intervening solvent or other neutral molecule) is designated as a 'tight ion pair' (or 'intimate' or 'contact ion pair'). A tight ion pair of X⁺ and Y⁻ is symbolically represented as X⁺Y⁻. By contrast, an ion pair whose @C01281@ ions are separated by one or several solvent or other neutral molecules is described as a 'loose ion pair', symbolically represented as X⁺ || Y⁻. The members of a loose ion pair can readily interchange with other free or loosely paired ions in the solution. This interchange may be detectable (e.g. by @I03345@) and thus afford an experimental distinction between tight and loose ion pairs. A further conceptual distinction has sometimes been made between two types of loose ion pairs. In 'solvent-shared ion pairs' the ionic constituents of the pair are separated by only a single solvent molecule, whereas in 'solvent-separated ion pairs' more than one solvent molecule intervenes. However, the term 'solvent-separated ion pair' must be used and interpreted with care since it has also widely been used as a less specific term for 'loose' ion pair.