Which one of the following substances crystallizes as a molecular solid?

• A. CO2
• B. NaCl
• C. SiO2
• D. Fe

Answer: (A)

Verified step by step guidance

1. Understand the types of solids: Ionic solids (like NaCl) consist of ions held together by ionic bonds; covalent network solids (like SiO2) have atoms connected by a continuous network of covalent bonds; metallic solids (like Fe) consist of metal atoms sharing a 'sea' of electrons; molecular solids are composed of molecules held together by intermolecular forces.
2. Identify the nature of each substance: NaCl is an ionic compound, SiO2 forms a covalent network solid, Fe is a metallic solid, and CO2 is composed of discrete molecules.
3. Recall that molecular solids are formed by molecules held together by relatively weak intermolecular forces such as London dispersion forces, dipole-dipole interactions, or hydrogen bonds, rather than by ionic or covalent bonds extending throughout the solid.
4. Recognize that CO2 crystallizes as a molecular solid because it consists of individual CO2 molecules held together in the solid state by intermolecular forces, not by ionic or covalent networks.
5. Conclude that among the given options, CO2 is the substance that crystallizes as a molecular solid.