Allotropes are defined as...

• A. Different chemical compounds made of the same element
• B. The same substance existing in different phases
• C. Different crystal forms of the same element
• D. Allotropes refer to isotopes of elements

Answer: (C)

Allotropes arise when a single element can adopt more than one arrangement of its atoms in the solid, giving different crystal structures and properties. This is exactly what “different crystal forms of the same element” means. For example, carbon exists as diamond and graphite, which are both pure carbon but arranged differently in the solid lattice. Oxygen also has distinct forms, like O2 and O3, which are different crystal/molecular forms of the same element. These are not different chemical compounds (which would involve different elements or bonds), nor are they simply the element in different phases (solid, liquid, gas) or isotopes (which differ in neutron number). So the correct description is that allotropes are different crystal forms of the same element.