Silicon (Si): Properties, Reactions & Applications

Introduction

Silicon (Si) is the second most abundant element in the Earth’s crust, constituting approximately 28% by mass. It is a metalloid with properties intermediate between metals and non-metals. Primarily found in its oxidized form as silicon dioxide (silica) and silicates, silicon forms the bedrock of geology, construction, and modern technology. Its unique electronic properties make it indispensable in semiconductor industries, powering the digital age.

CBSE/JEE Quick Revision Notes

• Atomic Number: 14
• Atomic Mass: 28.085 u
• Symbol: Si
• Group: 14 (Carbon family, p-block element)
• Period: 3
• Nature: Metalloid
• Valency: 4 (tetravalent)
• Electronegativity (Pauling): 1.90
• Common Oxidation States: +4, +2 (less stable), -4 (in silicides)
• Allotropy: Exists in amorphous (brown powder) and crystalline (grey, metallic lustre) forms.
• Bonding Preference: Predominantly forms strong covalent bonds.

Electron Configuration & Bonding Behavior

Silicon’s electron configuration is 1s² 2s² 2p⁶ 3s² 3p² or [Ne] 3s² 3p². It has four valence electrons in its outermost shell (n=3).

1. Covalent Bonding: Due to its moderate electronegativity and four valence electrons, silicon primarily forms covalent bonds. In most compounds, it exhibits a +4 oxidation state, forming four single covalent bonds, often adopting a tetrahedral geometry (e.g., in SiO₂, SiCl₄).
2. Network Structures: Silicon tends to form extended covalent networks. For instance, in its elemental crystalline form, each silicon atom is tetrahedrally bonded to four other silicon atoms, leading to a diamond-like structure.
3. Si-O Bonds: Silicon’s tendency to form strong bonds with oxygen (Si-O-Si) is a defining characteristic, leading to the stability and abundance of silicates and silica. This bond strength is significantly greater than C-O bonds due to the availability of empty d-orbitals in silicon for back-bonding with oxygen’s lone pairs, and the larger size difference allowing for better orbital overlap in certain contexts.
4. Absence of Multiple Bonds: Unlike carbon, silicon does not readily form stable pπ-pπ multiple bonds (e.g., Si=Si, Si=O) due to the larger size of its 3p orbitals, which results in poor lateral overlap.

Crucial Chemical Reactions

1. Reaction with Oxygen: At high temperatures, silicon readily reacts with oxygen to form silicon dioxide. Si(s) + O₂(g) → SiO₂(s)

2. Reaction with Halogens: Silicon reacts vigorously with halogens to form tetrahalides. Si(s) + 2Cl₂(g) → SiCl₄(l) (Silicon tetrachloride)

3. Reaction with Strong Bases (Alkalis): Silicon dissolves in hot concentrated alkali solutions, producing silicates and liberating hydrogen gas. Si(s) + 2NaOH(aq) + H₂O(l) → Na₂SiO₃(aq) + 2H₂(g) (Sodium silicate)

4. Formation of Magnesium Silicide: Silicon can react with reactive metals like magnesium to form silicides, which are precursors to silanes. 2Mg(s) + Si(s) → Mg₂Si(s)

5. Preparation of Silane from Magnesium Silicide: Magnesium silicide reacts with acids to produce silanes (silicon hydrides). Mg₂Si(s) + 4HCl(aq) → 2MgCl₂(aq) + SiH₄(g)

Industrial and Biological Importance

Industrial Importance

1. Semiconductors: Ultrapure silicon is the foundational material for integrated circuits (microchips), transistors, diodes, and photovoltaic cells (solar cells). Its semiconducting properties allow for precise control of electrical conductivity.
2. Alloys:
   • Ferrosilicon: An alloy of iron and silicon, primarily used in steel manufacturing as a deoxidizer and an alloying agent to improve strength and elasticity.
   • Silicon-Aluminum Alloys: Light and strong, used in engine blocks and aerospace components.
3. Silicones: A class of synthetic polymers containing silicon-oxygen backbones (-Si-O-Si-) with organic groups attached to silicon atoms. They are used as lubricants, sealants, adhesives, medical implants, cosmetics, and water-repellents due to their thermal stability, chemical inertness, and hydrophobicity.
4. Ceramics and Glass:
   • Silica (SiO₂): The primary component of sand, quartz, and glass. High-purity silica glass is used in optical fibers.
   • Silicon Carbide (SiC): An extremely hard material, used as an abrasive, in refractory materials, and in high-temperature semiconductor devices.
5. Construction: Silicon compounds are critical components of cement, concrete, bricks, and ceramics.

Biological Importance

1. Structural Element: Silicon is an essential trace element for some lower organisms, such as diatoms and sponges, where it forms hydrated silica skeletons and structural components.
2. Connective Tissue: In higher animals and humans, silicon is present in connective tissues, bones, and cartilage. While its precise biochemical role in human physiology is still an active area of research, it is thought to play a role in bone mineralization and collagen formation. It is not currently classified as a universally essential nutrient for human survival in the same way as elements like calcium or iron.