Formation of oxides and chlorides The table below summarises the most common oxides and chlorides of Period 3 elements. Element Oxide Chloride Na Na2O NaCl Mg MgO MgCl2 Al Al2O3 AlCl3 Si SiO2 SiCl4 P P2O5 PCl3 S SO2 SCl2 Cl Cl2O Cl2 Oxides Most metallic elements, both metals and non-metals, combine directly with oxygen. Metallic elements form ionic oxides whilst non-metallic elements form covalently bonded oxides. For example: 2Mg(s) + O2(g) → 2MgO(s) Chlorides There are a number of methods for preparing chlorides. The direct preparation can be used although the reaction can be vigorous. For example: 2Al(s) + 3Cl2(g) → 2AlCl3(s)