Silicon (Si) is a metalloid located in Group 14 of the periodic table, with an atomic number of 14 and an atomic mass of 28.085 g/mol. It is the second most abundant element in the Earth’s crust after oxygen and serves as a fundamental component of minerals, rocks, and technological materials. Due to its semiconductor properties, it holds strategic importance in electronic and industrial applications.

Discovery

Silicon was first isolated in its pure form in 1823 by the Swedish chemist Jöns Jacob Berzelius through the reduction of potassium fluorosilicate with metallic potassium. Antoine Lavoisier had previously predicted the existence of the element but was unable to obtain it in pure form. Berzelius’s work laid the foundation for silicon chemistry.

Classification and Fundamental Properties

Silicon is classified as a metalloid, exhibiting intermediate properties between nonmetals and metals. It belongs to the carbon group (Group 14) in the periodic table and has four valence electrons. Its electron configuration is [Ne] 3s²3p². The atomic radius is 111 pm, the covalent radius is 111 pm, and the van der Waals radius is 210 pm. In its crystal structure, silicon atoms are arranged in a tetrahedral network of covalent bonds.

Crystal Structure and Semiconductor Properties

Silicon adopts a diamond-type lattice structure in which each atom forms covalent bonds with four neighboring silicon atoms. This configuration maintains an electronic band gap of approximately 1.12 eV, making silicon a semiconductor. It exists in monocrystalline and polycrystalline forms; monocrystalline silicon is preferred in electronic devices, whereas polycrystalline silicon is widely used in solar cells. Electrical conductivity is controlled through doping: Group 15 elements such as phosphorus produce n-type semiconductors, while Group 13 elements such as boron produce p-type semiconductors.

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Physical and Chemical Properties

Silicon is a hard, brittle solid with a gray metallic luster and a Mohs hardness of about 7. It has a melting point of 1414 °C and a boiling point of 3265 °C. Its density is 2.33 g/cm³. Silicon exhibits a thermal conductivity of 149 W/(m·K), which is higher than that of glass and ceramics. Chemically, it does not readily react with most acids and bases at room temperature but dissolves in hydrofluoric acid. At elevated temperatures, it combines with oxygen to form silicon dioxide (SiO₂) and reacts directly with halogens to produce compounds such as silicon tetrachloride (SiCl₄).

Thermodynamic and Mechanical Properties

Silicon has a specific heat capacity of 0.705 J/(g·K). Due to its low thermal expansion coefficient, it exhibits high dimensional stability under temperature fluctuations. Its elastic modulus ranges between 130 and 185 GPa. With a refractive index of 3.42, silicon is an important material for infrared optical applications.

Electronegativity and Reactivity

Silicon has a Pauling electronegativity of 1.90, which allows it to form compounds with both covalent and partially ionic character. Unlike carbon, it is less inert and readily reacts at high temperatures with oxygen, nitrogen, halogens, and certain metals. Through direct reactions, it forms silicon nitride (Si₃N₄), silicon carbide (SiC), and silicon halides.

Isotopes

There are three stable silicon isotopes in nature: ^28Si (92.23%), ^29Si (4.67%), and ^30Si (3.10%), all of which share similar physical and chemical properties. Additionally, laboratory-produced radioactive isotopes, such as ^32Si, exist but have short half-lives.

Natural Occurrence and Compounds

Silicon constitutes about 27% of the Earth’s crust, making it the second most abundant element after oxygen. It is not found in its free state but occurs in nature as quartz, feldspar, mica, clay minerals, and various silicate compounds. Its most common compound is silicon dioxide (SiO₂), while silicate minerals form the primary components of rocks and soils.

Silicon Cycle in the Earth’s Crust

Silicon enters natural cycles through the chemical weathering and dissolution of rocks. The decomposition of silicates contributes to the formation of sedimentary rocks and soils. Dissolved silicon in rivers and oceans is utilized by organisms such as diatoms, integrating it into biological processes.

Special Compounds in Silicon Chemistry

Silicon combines with hydrogen to form silanes (SiH₄ and its derivatives), with oxygen to form siloxane chains and silicates. Among organosilicon compounds, silicone oils, resins, and elastomers are of great industrial importance. Silicon tetrachloride (SiCl₄) and silanes are used in the production of silicones and in chemical vapor deposition (CVD) processes.

Biological Role and Importance for Living Organisms

Silicon is not an essential element for most living organisms but contributes to structural integrity in certain species. Aquatic organisms such as diatoms and sponges strengthen their cell walls and skeletal structures with silicon compounds. In plants, silicon enhances stress tolerance and supports cell wall structure. In humans, it is suggested to have indirect effects on connective tissue and bone development.

Environmental and Ecological Effects of Silicon

Silicon in soil enhances plant resistance to environmental stresses such as drought and salinity. In marine ecosystems, silicon influences phytoplankton production and, consequently, the base of the food chain. The silicon cycle plays a critical role in biological productivity and ecosystem balance.

Industrial Production of Silicon

On an industrial scale, silicon is produced through the carbothermic reduction of quartz (SiO₂) with coke at high temperatures. For semiconductor applications, the Czochralski method is used to grow high-purity monocrystals, which are the primary material for silicon wafers in electronic devices.

Silicon in Nanotechnology

In nanotechnology, silicon nanowires and nanoparticles are applied in optoelectronic and biosensor technologies. Due to their high surface area, band gap engineering capabilities, and tunable electrical properties, they offer new possibilities in medicine, energy storage, and photonics.

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Applications

Silicon is the fundamental material of the electronics industry due to its semiconductor properties, being essential for the production of microchips, solar cells, and transistors. It is also a key raw material in the manufacture of glass, ceramics, cement, and silicone polymers. In metallurgy, it is used as an additive to enhance the hardness and durability of steel and aluminum alloys.