Which of the following is a covalent solid ?

\(SiC\)

The correct answer is option 3. \(SiC\).

Let us delve into the details of each type of solid to understand why \(SiC\) (Silicon Carbide) is a covalent solid:

1. \(ZnS\) (Zinc Sulfide)

Type of Solid: Ionic Solid

Zinc sulfide forms an ionic lattice structure. In this lattice, zinc ions (\(Zn^{2+}\)) and sulfide ions (\(S^{2-}\)) are arranged in a regular pattern, held together by strong electrostatic forces of attraction between the positively and negatively charged ions. Ionic solids typically have high melting and boiling points, are hard and brittle, and conduct electricity only when molten or dissolved in water.

2. \(Ag\) (Silver)

Type of Solid: Metallic Solid

Silver has a metallic bonding structure where atoms are arranged in a closely packed lattice, and the bonding is characterized by a "sea of electrons" that are free to move around the lattice of positively charged metal ions. Metallic solids are characterized by their high electrical and thermal conductivity, malleability, ductility, and high melting and boiling points.

3. \(SiC\) (Silicon Carbide)

Type of Solid: Covalent Network Solid

Silicon carbide has a covalent network structure where each silicon atom is covalently bonded to four carbon atoms in a tetrahedral arrangement, and each carbon atom is covalently bonded to four silicon atoms, creating a three-dimensional network. The bonding in \(SiC\) involves a network of strong covalent bonds throughout the entire solid, rather than discrete molecules or ions. Covalent network solids like \(SiC\) are known for their very high melting points, hardness, and chemical resistance. They do not conduct electricity well as they lack free-moving charged particles, unlike metals.

4. \(CCl_4\) (Carbon Tetrachloride)

Type of Solid: Molecular Solid

Carbon tetrachloride forms molecular solids in its solid state. The individual \(CCl_4\) molecules are held together by intermolecular forces (van der Waals forces), not by covalent bonds in a lattice. Molecular solids typically have lower melting and boiling points compared to ionic or covalent network solids and do not conduct electricity. They are usually soft and can be easily melted or evaporated.

Summary:

Ionic Solids: Involve electrostatic attraction between ions (e.g., \(ZnS\)).

Metallic Solids: Feature a lattice of metal cations surrounded by a sea of delocalized electrons (e.g., \(Ag\)).

Covalent Network Solids: Consist of a network of covalently bonded atoms extending throughout the material (e.g., \(SiC\)).

Molecular Solids: Composed of discrete molecules held together by intermolecular forces (e.g., \(CCl_4\)).

Conclusion:

Silicon Carbide (\(SiC\)) is a covalent network solid because it consists of a three-dimensional network of strong covalent bonds between silicon and carbon atoms. This structure makes it distinct from ionic, metallic, and molecular solids.