Understanding Molecular Solids

Solids can be classified into different types based on the nature of the constituent particles and the forces holding them together. The main types are ionic solids, covalent or network solids, metallic solids, and molecular solids.

Identifying a Molecular Solid

A molecular solid is a solid in which the constituent particles are discrete molecules. These molecules are held together in the solid state by relatively weak intermolecular forces, such as van der Waals forces (London dispersion forces, dipole-dipole forces) and hydrogen bonds.

Let's examine the given options:

• Silica (\(\text{SiO}_2\)): Silica is silicon dioxide. In silica, silicon and oxygen atoms are linked by strong covalent bonds forming a continuous three-dimensional network structure. It is classified as a network covalent solid, not a molecular solid.
• Calcium fluoride (\(\text{CaF}_2\)): Calcium fluoride is an ionic compound formed between the metal calcium (\(\text{Ca}\)) and the non-metal fluorine (\(\text{F}\)). It consists of calcium ions (\(\text{Ca}^{2+}\)) and fluoride ions (\(\text{F}^{-}\)) held together by strong electrostatic forces of attraction (ionic bonds). It is an ionic solid.
• Methane (\(\text{CH}_4\)): Methane is a simple molecule consisting of one carbon atom covalently bonded to four hydrogen atoms. In solid methane, discrete \(\text{CH}_4\) molecules exist and are held together by weak intermolecular forces (specifically, London dispersion forces, as methane is a nonpolar molecule). Therefore, methane is a molecular solid.

Comparing the options, methane fits the definition of a molecular solid because it consists of discrete molecules held together by intermolecular forces.

Here is a summary of the types of solids for the given options:

Based on the analysis, methane is the molecular solid.