The absence of free electrons in their structures is what generally makes ionic solids poor conductors of electricity. Although ions are electrically conductive when they are unbound, ionic solids exist in crystalline form, with each ion in a relatively fixed position surrounded by ions of the opposite charge.
This poor electrical conduction is complemented by poor heat conduction. Thermal vibrations find difficulty moving from one end of the ionic solid to the other because of the fixed position of the ions. Hardness and brittleness are two other customary traits of ionic solids. These mechanical properties manifest because the planes of ions in an ionic crystal are not free to glide over one another. Any attempt to mechanically displace these planes, though either tension, compression, twisting or impact, may result in two similarly charged atoms in adjacent planes meeting and violently repelling one another, causing the ionic solid to fracture. Some ionic solids, such as sodium chloride, are water-soluble. When these solids come in contact with polar solvents, such as water, their lattice ions are attracted to the molecules of the solvent. These solvent molecules carry the crystal ions away from the bulk of the crystal, gradually causing it to dissolve. Stirring or heating speeds this process up.