Which of the following is NOT one of the four types of crystalline solids?

• A. Ionic Solids
• B. Molecular Solids
• C. Covalent Solids
• D. Amorphous Solids

Answer: (D)

Crystalline solids are defined by a repeating long-range arrangement of atoms or ions. There are four common categories based on bonding and structure: ionic solids, molecular solids, covalent (network) solids, and metallic solids. Amorphous solids, by contrast, lack this long-range order and do not form a regular periodic lattice. They may have some short-range order, but not the extended periodic structure that characterizes crystalline materials. That’s why amorphous solids are not considered crystalline, making them the right choice here. For context, examples include: ionic solids like NaCl, molecular solids like solid CO2, covalent solids like diamond, and metallic solids like iron. An amorphous example is glass.