Silicon | Research & Encyclopedia Articles

Silicon

Silicon is the second element in Group 14 of the periodic table. It has an atomic number of 14, an atomic mass of 28.0855, and a chemical symbol of Si

Properties

Silicon exists in two allotropic forms, one of which consists of shiny, grayish black needle-like or crystal plates. The other allotrope is an amorphous brown powder. The melting point of the crystalline allotrope is 2,570°F (1,410°C), its boiling point is 4,270°F (2,355°C), and its density is 2.33 grams per cubic centimeter. Silicon is a relatively hard element with a hardness of 7 on the Mohs scale. Silicon is a semiconductor, a property which determines some of its most important uses.

As its two allotropic forms might suggest, silicon is a metalloid. It is relatively inactive at room temperature, and resists attack by water and most acids. At higher temperatures, it reacts with many metals, oxygen, nitrogen, sulfur, phosphorus, and the halogens. It also forms a number of alloys in the molten state.

Occurrence and Extraction

Silicon is the second most abundant element in the Earth's crust, ranking second only to oxygen. It has an abundance estimated to be about 27.6%. According to some experts, as much as 97% of the Earth's crust may be made of rocks that contain silicon, oxygen, and one or more other elements. Silicon has also been detected in the Sun and stars and is found in certain types of meteorites known as aerolites, or "stony meteorites." Silicon never occurs free in nature, but is usually found as a compound with oxygen, magnesium, calcium, phosphorus, and/or other elements. The most common minerals are those that contain silicon dioxide (SiO₂) in one form or another. Compounds that contain silicon and oxygen with one or more other elements are known as silicates.

Silicon is extracted from its ores by heating silicon dioxide with carbon: SiO₂ + C --heat CO₂ + Si. Many situations in which silicon is used require that the element be very pure. Silicon with a purity of more than 99.97% is called hyperpure silicon.

Discovery and Naming

The discovery of silicon as an element evaded chemists for many years because of the stability of most silicon compounds. Chemists had little reason to believe that a new element existed in sand, silicates, and other earthy materials, nor did they have the technology to extract the element from its compounds. One researcher with perhaps the greatest reason to hope for success in producing silicon was the English chemist and physicist Humphry Davy. Davy had developed a technique by which unusually stable compounds could be decomposed into their constituent elements. He used this method to prepare sodium, potassium, calcium, and other elements for the first time. He was unsuccessful, however, in producing silicon by the same method.

The first successful effort in the search for silicon was achieved by the Swedish chemist Jöns Jakob Berzelius. In 1823, Berzelius electrolyzed a molten mixture of potassium metal and potassium silicon fluoride (K₂SiF₆) and obtained a small sample of pure silicon: 4K + K₂SiF₆ --heat and electricity 6KF + Si. The new element was named by the Scottish chemist Thomas Thomson (1773-1852) because of the element's presence in the mineral flint (silex or silicis in Latin). He added the ending -on because of the element's similarity to carbon.

Uses

Probably the best known use of silicon is in transistors, photovoltaic cells, rectifiers, and other electronic devices. The largest single use of the element, however, is in the production of alloys, especially various forms of steel. The process by which iron is extracted from its ores always results in a product in which iron, silicon, and other elements are present in the form of an alloy. The manufacture of various types of steel involves primarily the enhancement or removal of one or more of these components.

One of the most common silicon-iron alloys, for example, is ferrosilicon. This alloy is used for two major purposes. First, it can be added to other forms of steel to improve their strength and toughness. Second, it can be added during the steel-making process to remove impurities from the steel that is being made. The aluminum industry also produces a number of silicon alloys. These alloys are used primarily to make molds and in the welding process. Alloys of silicon, aluminum, and magnesium are very resistant to corrosion and are used in the construction of large buildings, bridges, and transportation vehicles, such as ships and trains.

Hundreds of silicon compounds are used for a variety of practical applications. The most common silicon compound, silicon dioxide, is used in the manufacture of glass, ceramics, abrasives, as a food additive, in water filtration systems, as insulating material, in cosmetics and pharmaceuticals, and in the manufacture of paper, rubber, and insecticides. A compound known as silicon carbide (SiC), also known as carborundum, is one of the hardest substances known. It is used as a refractory material and as an abrasive. Another category of silicon compounds are the silicones--compounds containing silicon, oxygen, and one or more organic groups. Silicones are used to make toys (such as Silly Putty and Superballs), lubricants, weatherproofing materials, adhesives, foaming agents, brake fluids, cosmetics, polishing agents, electrical insulation, surgical implants, and parts for automobile engines.