Which element is known to have white, red, and black allotropes?

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

Answer: (C)

Allotropy is when an element can exist in more than one structural form with different properties. Phosphorus shows white, red, and black allotropes because its atoms can bond in distinct ways, creating forms with very different structures and stabilities. White phosphorus is made of P4 tetrahedra and is highly reactive, often kept under water to prevent ignition. Red phosphorus has a more networked, polymer-like structure, which makes it less reactive and more stable. Black phosphorus has a layered, puckered arrangement that is the most stable and has semiconducting properties. These differences among the phosphorus forms explain why the white, red, and black allotropes are each named and studied. Other elements don’t typically present this exact trio of allotropes—oxygen has O2 and O3, carbon has diamond and graphite (among others), and nitrogen is mainly known for N2 with other forms not standardly colored as white, red, or black.