The most common silicone compound, poly-dimethylsiloxane, can illustrate the central characteristics of the class. The starting material is metallic silicon, which is obtained from silica sand. Silicon is reacted with methyl chloride (CH 3 Cl) over a copper catalyst, forming dimethyldichlorosilane ([CH 3] 2 Si[Cl] 2). Silicon. Silicon is a brittle steel-gray metalloid. In elemental form it has the same structure as diamond, each silicon forming single covalent bonds with four adjacent silicon atoms which surround it tetrahedrally. Silicon chemistry is a vast area of study. Silicon is a constituent of many of the minerals which make up the earth's crust. Silicon is passivated by a thin layer of SiO 2 on the surface, and does not otherwise react with air under normal conditions. If heated above 900 °C, silicon reacts with the oxygen, O 2, forming SiO 2: Si (s) + O 2 (g) SiO 2 (s) If heated above 1400 °C, silicon reacts with the nitrogen, N 2, forming the silicon nitrides SiN and Si 3 N 4: 2 Si.