If the binding forces are strong, the crystal will have a high melting point. If they are somewhat weaker, the crystal will have a lower melting point and may be easily bent and shaped. If they are very weak, the crystal will form only at very low temperatures, when not much energy is available to the molecules. There are four main types of crystal bonds. Ionic crystals are made up of positive and negative ions that are bound together by attraction of opposite electrical charges.
Sodium chloride is an example of an ionic crystal.
The atoms or molecules of covalent crystals share their valence electrons equally. Diamonds, germanium, and silicon are important covalent crystals. The individual atoms of metals become positive ions surrounded by free valence electrons that can move easily from atom to atom. When these free electrons all move in the same direction, their movement is called an electrical current. The molecules of molecular crystals do not share their electrons at all.