Ionic solids are insulators. Since ions in solids are tightly bound, they cannot conduct electricity. However, When the solid is dissolved in water, the separated ions can freely move and conduct electricity through the solution. The strong attraction between negative and positive ions causes ionic solids to harden. An ionic bond is formed when non-metal reacts with a metal. Common examples of ionic compounds include sodium chloride and magnesium oxide, or NaCi and MgO.