Ionic crystals are compounds of elements held together by ionic bonds. The compound must have at least two components which are ionized, although many have several. Bonds are quite strong, usually electrostatic, at 1000kj per molecule. There are no free electrons. As crystals, they from regulated shapes either simple cubes or complex forms including the poly faceted forms of tetrahedral and octahedral forms. The more complex types of compounds are those with multiple ionic elements as components. Ionic crystals are generally lattice structures, very hard and with high melting points. These substances are also known as ionic solids.