Which allotropic form of phosphorus is the most stable?

Phosphorus exists in several allotropic forms, including white, red, violet, and black phosphorus, each differing in physical appearance, crystal structure, and stability. The most stable allotrope under standard temperature and pressure is black phosphorus.

Black phosphorus is formed when white phosphorus is heated under high pressure or sometimes in the presence of a catalyst like iodine. It has a layered, crystalline structure similar to graphite, where each phosphorus atom is covalently bonded to three other atoms, forming puckered sheets. These layers are held together by van der Waals forces, giving the structure both strength and flexibility.

This configuration makes black phosphorus thermodynamically stable and chemically less reactive than other allotropes. It does not ignite spontaneously in air, unlike white phosphorus, which is highly reactive and can catch fire at room temperature due to its strained tetrahedral P₄ molecules.

Red phosphorus, on the other hand, is more stable than white phosphorus but still less stable than black phosphorus. It is commonly used in safety matches, fireworks, and flame retardants because of its moderate reactivity. Violet phosphorus (Hittorf’s phosphorus) is an intermediate form, possessing properties between red and black phosphorus but is rarely encountered in practical applications.

In contrast, white phosphorus is the least stable and most reactive allotrope, stored under water to prevent oxidation. It is toxic and emits a faint glow in the dark due to slow oxidation (chemiluminescence).

Therefore, the most stable allotropic form of phosphorus is black phosphorus, known for its unique layered structure, high stability, and semiconducting properties, making it an interesting material for modern electronic applications