For example, in sodium chloride, Cl- ions have a definite regular arrangement. In this arrangement, each Na+ is surrounded by six Cl- ions and each Cl- is surrounded by six Na+ ions. Similar regular arrangements are also found in other ionic solids.
Fig: 6.14 - Structure of sodium chloride
Physical characteristics of ionic solids
- Ionic solids are hard and brittle.
- Ionic solids dissolve in polar solvents like water "The ions of the ionic solids are pulled out of their lattice sites by the dipole movement of the polar solvent and dispersed into solvent form in the solution.
- Ionic solids have high melting and boiling points. To overcome the strong electronegative forces that hold their ions together, considerable thermal energy has to be applied to melt ionic solids. During melting, oppositely charged ions of the destroyed crystal tend to stay together, thus molten ionic solids posses a quasilattice structure.
- The ions in the molten stage of an ionic substance are free to move as opposed to being fixed to their lattice sites as ionic solids. Therefore ionic solids do not conduct electricity in a solid state but do so only in the molten state.
- Ionic solids are anisotropic in nature. This means that properties like mechanical strength, conductivity, refractive index etc. depend upon the direction along which they are measured.
- Ionic solids have negligible vapour pressure at room temperature.
The arrangement of ions in a particular ionic solid depends upon:
- The relative sizes of the positive and negative ions.
- Magnitude of the charges on the ions.