Allotropy refers to which of the following phenomena?

• A. When an element exists in more than one crystal structure
• B. When two elements form a compound with different stoichiometry
• C. When a compound changes composition with temperature
• D. When a metal forms a single crystal

Answer: (A)

Allotropy is the phenomenon where an element can exist in more than one crystal structure, meaning the same element has different atomic arrangements in the solid without changing its chemical makeup. That’s why this description fits: an element that can adopt more than one crystal structure. Classic examples include carbon, which can be diamond (tightly bonded network) or graphite (layered sheets), both pure carbon but with very different properties; phosphorus also has several allotropes (white, red, black); iron shows different crystal structures depending on temperature (ferrite and austenite are different lattices of the same element). The other ideas describe changing which compounds are formed or how a crystal grows, not different structures of the same element, so they aren’t about allotropy.