Which of the following is a molecular solid?

\(I_2\)

The correct answer is option 4. \(I_2\).

Let us delve into the nature of each type of solid and explain why \(I_2\) is classified as a molecular solid:

Ionic Solids:

\(ZnS\) (Zinc Sulfide)

In ionic solids, such as \(ZnS\), the structure consists of positively charged cations and negatively charged anions arranged in a regular, repeating pattern to maximize ionic interactions and minimize electrostatic repulsion. For \(ZnS\), zinc ions (\(Zn^{2+}\)) and sulfide ions (\(S^{2-}\)) form a lattice.

Bonding: The bonding in ionic solids is electrostatic, with strong ionic bonds holding the lattice together. These interactions result in high melting points, hardness, and brittleness.

Properties: Ionic solids are typically hard and brittle with high melting points. They are also poor conductors of electricity in the solid state but can conduct when molten or dissolved in water.

Covalent Network Solids:

Diamond

Covalent network solids like diamond consist of a three-dimensional network where each atom is covalently bonded to its neighbors. In diamond, each carbon atom forms four covalent bonds with adjacent carbon atoms, creating a very strong and rigid structure.

Bonding: The bonds are strong covalent bonds throughout the entire structure, resulting in very high melting points and hardness. The extensive network of bonds makes these solids very durable.

Properties: These solids generally have high melting points, are very hard, and do not conduct electricity (except for some forms of graphite). They are also typically insoluble in most solvents

\(SiC\) (Silicon Carbide)

Similar to diamond, silicon carbide features a covalent network structure where silicon and carbon atoms are covalently bonded in a continuous three-dimensional lattice.

Bonding: The bonding is covalent, which contributes to its hardness and high melting point.

Properties: \(SiC\) is very hard and has a high melting point. It is used in abrasives and cutting tools.

Molecular Solids:

\(I_2\) (Iodine)

Molecular solids consist of discrete molecules held together by weak intermolecular forces such as van der Waals forces (dispersion forces), dipole-dipole interactions, or hydrogen bonds. In \(I_2\), the structure consists of individual iodine molecules (\(I_2\)) arranged in a lattice.

Bonding: The primary interactions between the molecules are van der Waals forces, which are much weaker than ionic or covalent bonds.

Properties: Molecular solids generally have lower melting and boiling points compared to ionic and covalent network solids. They are often soft and can be more easily deformed. They also tend to be soluble in non-polar solvents.

In the case of \(I_2\):

Molecular Structure: Iodine forms a molecular solid where the \(I_2\) molecules interact through weak van der Waals forces.

Solubility: \(I_2\) is more soluble in non-polar solvents like hexane.

Summary:

Ionic solids (like \(ZnS\)) have ionic bonds in a crystalline lattice. Covalent network solids (like diamond and \(SiC\)) have an extensive network of covalent bonds throughout the material. Molecular solids (like \(I_2\)) have discrete molecules held together by weak intermolecular forces.

Thus, the molecular solid among the options given is: 4. \(I_2\).