Period 3 elements. From left (in their periodic table order) they are: sodium (Na); magnesium (Mg); aluminium (Al); silicon (Si); phosphorus (P, white/red); sulphur (S); chlorine (Cl); and argon (Ar). Successive elements across the period, have a single electron added, and this causes immense changes in the chemical and physical properties. Metals (Na/Mg/Al) become non-metals (P/S/Cl/Ar), solids become gases, and reactive elements (Na/Mg) become inert (Ar). Mg is used in flares, Ar in light bulbs, and Si in microchips. P and S have different structural forms (allotropes). Cl and Ar form diatomic (Cl2) and monatomic (Ar) molecules.