Which element has known allotropes including white, red, and black varieties?

• A. Oxygen
• B. Carbon
• C. Phosphorus
• D. Nitrogen

Answer: (C)

Allotropy is when an element can exist in different structural forms with distinct bonding and properties. Phosphorus shows several well-known allotropes that differ markedly in structure and reactivity: White phosphorus consists of P4 tetrahedra, a pale, waxy, highly reactive form that must be kept under water. Red phosphorus is formed by heating white phosphorus in air and consists of a polymeric network, making it less reactive and more stable. Black phosphorus has a layered, puckered structure akin to graphite and is the most stable allotrope, with interesting semiconducting properties. The specific trio white, red, and black is characteristic of phosphorus, setting it apart from the other elements listed, which do have allotropes but not in this same named set. Therefore phosphorus is the correct choice.