Understanding Carbon Allotropy

Allotropy is the property where an element can exist in two or more different forms in the same physical state. These different forms are called allotropes and differ in their atomic arrangement. Carbon exhibits allotropy, meaning it exists in various structural forms.

Analyzing the Options for Carbon Allotropes

Let's examine each option to determine if it's an allotrope of carbon:

• Chalk: Primarily composed of calcium carbonate, with the chemical formula $CaCO_3$. It is a sedimentary rock, not a pure form of carbon.
• Diamond: This is a well-known allotrope of carbon. In diamond, carbon atoms are arranged in a tetrahedral crystal structure, making it extremely hard.
• Gypsum: This mineral is hydrated calcium sulfate, chemically represented as $CaSO_4 \cdot 2H_2O$. It is not related to carbon allotropes.
• Marble: A metamorphic rock primarily made of calcium carbonate ($CaCO_3$). Similar to chalk, it's a carbonate mineral, not an allotrope of elemental carbon.

Identifying the Correct Allotrope

Based on the analysis, Diamond is the only substance listed that represents a distinct structural form (allotrope) of the element carbon.