= Ionic bonding
{wiki}
<Ionic bonding> binds oppositely charged <ions> through attraction governed by <electrostatics>. In ordinary solid <sodium chloride>, each <cation> interacts with many <anions>, and also with other <cations>: attraction and repulsion together contribute to the binding of an extended <ionic crystal>. The <chemical formula> $\mathrm{NaCl}$ specifies equal numbers of the two kinds of <ions>; it does not divide the <solid> into isolated two-atom <molecules>.<Chemistry 2e>[§7.1, “The Formation of Ionic Compounds”, Figure 7.3]{ref}
This distinction explains a striking combination of properties: ordinary dry <sodium chloride> conducts <electric current> poorly, while its molten form or a sufficiently concentrated <chemical solution> in <water> conducts well. The <ions> already exist in the <solid>; their ability to move through the sample changes. A <conductivity experiment on salt solutions> tests this transport picture, rather than simply asserting that charged constituents must make every material conductive.
\Image[/ionic-crystal-pair-and-binding-energy.png]
{title=An isolated <ion pair>, a plane of an extended <ionic crystal>, and a synthetic <potential energy> model with a stable spacing}
{id=image-ionic-crystal-pair-and-binding-energy}
{description=Original schematic geometry and calculated model curves, not a measured <electron density>, <diffraction> pattern, or experimental <lattice energy>. Four nearest neighbours are visible in the plane of the <rock salt structure>; two more lie perpendicular to it. Disk sizes are arbitrary. The right-hand curves use the explicitly stipulated <dimensionless> model below.}
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