Which type of solid is $SiO_2$?

Covalent Solid

The correct answer is Option (4) → Covalent Solid

Let us delve deeper into why \(SiO_2\) (silicon dioxide) is classified as a covalent solid, exploring its structure, properties, and comparisons with other types of solids

Structure of \(SiO_2\)

Covalent Network Solid:

\(SiO_2\) has a covalent network structure, which means it consists of a large number of atoms bonded together by covalent bonds in a continuous network.

Each silicon atom is tetrahedrally coordinated to four oxygen atoms, while each oxygen atom is bonded to two silicon atoms. This results in a three-dimensional arrangement:

\(\text{Si} \longrightarrow \text{O} \longrightarrow \text{Si} \longrightarrow \text{O} \ldots\)

This bonding creates a strong, stable structure, similar to diamonds (another covalent solid), where each atom is connected to many others.

Properties of \(SiO_2\)

High Melting and Boiling Points: Due to the strength of the covalent bonds that hold the atoms together, \(SiO_2\) has a high melting point (around 1710 °C). A significant amount of energy is required to break these strong bonds.

Hardness: \(SiO_2\) is very hard and is commonly found in nature as quartz. It’s used in many applications, including glass-making and construction materials, due to its hardness and durability.

Brittleness: Despite its hardness, \(SiO_2\) is brittle, meaning it can break or shatter under stress. This property is characteristic of covalent solids because the strong directional bonds can fracture when force is applied.

Insolubility: \(SiO_2\) is insoluble in water and does not conduct electricity in solid form because it does not have free electrons or ions that can move.

Comparison with Other Types of Solids

Molecular Solids: These solids consist of discrete molecules held together by weaker intermolecular forces (like Van der Waals forces). Examples include ice and sugar. They typically have lower melting points and are softer than covalent solids.

Ionic Solids: These consist of positively and negatively charged ions held together by strong electrostatic forces (ionic bonds). Examples include NaCl (table salt). Ionic solids usually have high melting points and conduct electricity when dissolved in water, but they are brittle.

Metallic Solids: These consist of metal atoms that share delocalized electrons in a "sea" of electrons, which allows them to conduct electricity and heat. They typically have varying melting points and are malleable and ductile.

Conclusion

\(SiO_2\) is classified as a covalent solid due to its strong covalent bonding in a network structure, leading to distinct physical properties like high melting and boiling points, hardness, and brittleness. Understanding these characteristics helps in distinguishing covalent solids from other types of solids, emphasizing the unique behavior and applications of materials like silicon dioxide.