Atoms are most stable if they have a full outer shell. Atoms bond with one another to achieve full outer shells - this is why most elements form compounds. Noble gases do not form compounds, since they already have full outer shells. Metal atoms (and hydrogen), such as sodium, lose electrons to become ions. Ions are charged particles. Since metals lose electrons to achieve full outer shell configurations, the metal ions have positive charges. A positive ion is called a cation. Non-metal atoms, such as chlorine, gain electrons and become ions. By gaining electrons they can achieve a full outer shell. Since they gain electrons non-metal ions have negative charges. A negative ion is called an anion. Metal atoms bond with non-metal ions by transferring their electrons. This is called ionic bonding. For some elements, the energy involved in losing or gaining electrons is too much. In these cases their atoms share electrons. This bonding associated with the sharing of electrons is called covalent bonding. There are four types of solid, giant ionic, giant covalent, metallic and simple molecular. Sodium chloride is an example of a giant ionic solid. It consists of oppositely charged ions held together by electrostatic forces. They are soluble in water and conduct electricity when molten or in solution. Diamond and graphite are examples of giant (covalent) molecular solids. They consist of millions of covalent bonds that cause them to have very high boiling points. Iodine is an example of a simple molecular solid. It has a low boiling point because molecules of I2 are held together by weak forces. They are insoluble in water and do not conduct electricity. Metals consist of tightly packed atoms whose outer electrons become part of a ' sea ' of electrons. This delocalisation of electrons explains why metals are good conductors of heat and electricity.