Allotropy is defined as the existence of an element in more than one crystal structure.

• A. A compound with different chemical formulas
• B. A phase that includes liquids and gases
• C. An element that forms different ions
• D. An element that exists in more than one crystal structure

Answer: (D)

Allotropy is the phenomenon where an element can exist in more than one crystal form. This is exactly what the statement captures: an element existing in more than one crystal structure. For example, carbon shows up as diamond and graphite, two different crystal lattices with very different properties. Phosphorus has several allotropes with distinct atomic arrangements as well. The other ideas describe things that aren’t about different crystal structures of the same element—compounds with different formulas, phases that include liquids or gases, or different ionic forms—so they don’t describe allotropy.