Silicon

Silicon (Si), with an atomic number of 14, is the second most abundant element in the Earth's crust.

It is a crystalline metalloid, meaning it exhibits both metallic and non-metallic properties. In nature, silicon is commonly found in its combined form as silicon dioxide, also known as silica.

Silicon plays a crucial role in various industries. It is a key ingredient in the production of cement, ceramics, glass, and most semiconductor devices. Additionally, it is an essential component in certain types of steel and bricks.

Glass manufacturing involves the use of sand (silica, SiO2), soda ash (Na2CO3), and limestone (CaCO3) as primary raw materials. Other additives, such as dolomite (CaMg(CO3)2), sodium sulfate (Na2SO4), and alumina (Al2O3), are also added to improve the properties of the final product.

Properties of Silicon

  1. It exists as a solid at room temperature and pressure.
  2. It is very brittle and has a metallic lustre.
  3. It is an intrinsic semiconductor in its pure form.
  4. One allotrope of silicon appears as shiny, greyish-black, needle-like crystals or flat plates, while another lacks crystal structure and appears as a brown powder.
  5. It is classified as a metalloid.
  6. It has a melting point of 1410°C and a boiling point of 3265°C.
  7. Its density is 2.33 g/cm3.
  8. It is relatively inactive at room temperature but becomes highly reactive at high temperatures.
  9. In its molten state, it combines with elements like oxygen, nitrogen, and phosphorus.
  10. It can form alloys when molten.
  11. It dissolves only in a mixture of nitric acid and hydrofluoric acid.

Uses of Silicon

  1. It is used to create alloys such as iron-silicon and aluminium-silicon.
  2. It is involved in the manufacture of glass, ceramics, bricks, and fire bricks.
  3. Due to its semiconductor properties, it is used in making transistors, solar cells, rectifiers, and other solid-state electronic devices.
  4. Silicone is utilized as sealants and electrical insulators.
  5. It is used in mould release agents and moulding compounds.
  6. Silicone rubber is used in waterproofing applications, such as bathrooms, pipes, and roofs.
  7. Silicon tetrachloride is used to produce ultrapure silicon for making transistors, computer chips, and solar cells.

Compounds/Polymers of Silicon

  1. Silanes:

    Silanes, also known as silicon hydrides, are compounds composed only of silicon and hydrogen with the general formula SinH2n+2 (where n = 1, 2, 3, etc.). They are produced by reacting magnesium silicide (Mg2Si) with acids or by reducing silicon chlorides using lithium aluminium hydride. Silanes are highly reactive in air and can ignite spontaneously. They break down in alkaline solutions to produce hydrogen and hydrated silica, and decompose into hydrogen and silicon upon heating. Silanes are strong reducing agents and are used in water repellents and sealants. The simplest and most stable silane is monosilane (SiH4).

  2. Silicides:

    Silicides share structural similarities with carbides and borides. Silicon carbide, also known as carborundum, is commonly used for grinding, polishing, and as a refractory material due to its hardness and thermal resistance.

  3. Halides:

    Silicon reacts readily with halogens to form silicon tetrahalides (SiX4, where X is a halogen). These halides are often used to purify silicon and are more reactive with water than carbon tetrahalides, undergoing hydrolysis easily.

  4. Silica Gel:

    Hydrated silica gel is formed when water concentration is increased. It is commonly used as a drying agent and in desiccants.

  5. Silicones:

    Silicones are synthetic polymers made from organosilicon compounds with repeating silicon-oxygen units. They are produced from alkyl-substituted silanes and are valued for their durability, thermal stability, resistance to oxidation and chemicals, and low volatility. Silicones are widely used in electrical insulation, sealants, and surgical applications.

  6. Silicates:

    Silicates are the most abundant form of silicon in nature. Silicon dioxide (SiO2), also known as silica, is found in granite and sandstone. Silicates are used in making glass, abrasives, food additives, and for water filtration.

Silica

Silica, also known as silicon dioxide (SiO2), is a covalent, three-dimensional network solid composed of one silicon atom bonded to four oxygen atoms. It forms a tetrahedral structure where each silicon atom creates four single bonds with surrounding oxygen atoms.

Unlike carbon dioxide, which forms double bonds, silica prefers to form single bonds due to the energy configurations of silicon’s 3p orbitals. This structure results in extended -Si-O-Si-O- networks known as silicates.

Although the molecular structure is complex, the empirical formula of silica is SiO2, as each silicon atom is bonded to two oxygen atoms on average. The SiO44− tetrahedral complex forms the basic unit of silicates. These units can link together in various ways, leading to a wide diversity of silicate minerals.

Just as carbon is the foundational element in organic compounds, silicon plays a critical role in the formation of minerals. Silica is essential in geological formations and widely used in manufacturing industries.

There are two notable amorphous forms of silica: