Silicon
. Silicon, symbol Si, semimetallic element that is the second most common element on earth, after oxygen . The atomic number of silicon is 14. Silicon is in group 14 (or IVa) of the periodic table . It was first isolated from its compounds in 1823 by the Swedish chemist Baron Jöns Jakob Berzelius . PROPERTIES AND OCCURRENCE Silicon is prepared as a brown amorphous powder or as gray-black crystals. It is obtained by heating silica, or silicon dioxide (SiO2), with a reducing agent, such as carbon or magnesium, in an electric furnace. Crystalline silicon has a hardness of 7, compared to 5 to 7 for glass. Silicon melts at about 1410° C (about 2570° F), boils at about 2355° C (about 4271° F), and has a specific gravity of 2.33. The atomic weight of silicon is 28.086. Silicon is not attacked by nitric, hydrochloric, or sulfuric acids, but it dissolves in hydrofluoric acid, forming the gas, silicon tetrafluoride. It dissolves in sodium hydroxide, forming sodium silicate and hydrogen gas. At o