Which statement best describes allotropy in metals under varying conditions?

• A. It never occurs in metals
• B. It is always accompanied by a chemical reaction
• C. It requires a change in composition
• D. It can involve a crystal-structure change with temperature or pressure

Answer: (D)

Allotropy is when an element can exist in more than one crystalline form in the same phase. In metals this occurs because changing conditions like temperature or pressure can rearrange the atoms into a different lattice without changing the element’s chemical composition. So, allotropy involves a crystal-structure change driven by temperature or pressure, not a chemical reaction or a change in what the element is made of. A classic example is iron, which shifts between different crystal structures as temperature changes, affecting its properties.