Which substance is a simple molecular lattice?

• A. Iodine
• B. Methane
• C. Sodium chloride
• D. Water

Answer: (A)

A simple molecular lattice is a solid made of discrete molecules held together in a regular arrangement by weak intermolecular forces, while the strong bonds exist inside each molecule. Iodine fits this description well: it forms solid composed of I2 molecules arranged in a lattice, with the covalent bonds inside each I2 molecule being strong, and the interactions between molecules (London dispersion forces) being comparatively weak. This gives iodine a molecular solid structure.

Water in ice is held together by hydrogen bonds between whole water molecules, creating a more extended network than a simple molecular lattice. Sodium chloride forms an ionic lattice with strong electrostatic attractions between oppositely charged ions, not discrete molecules. Methane can also form a molecular solid, but iodine is the classic example of a simple molecular lattice shown by its solid-state arrangement of intact molecules bound by weak intermolecular forces.