Question

The most thermodynamically stable allotropic form of phosphorus is _____.
A) Red
B) White
C) Black
D) Yellow

Answer 1

Hint: To solve this we must know that thermodynamic stability occurs when a system is in its lowest energy state or is in chemical equilibrium with its environment. Phosphorus has different forms red, white, black and yellow. We must know the most stable form of phosphorus.

Complete step-by-step answer:
We know that thermodynamic stability occurs when a system is in its lowest energy state or is in chemical equilibrium with its environment. Phosphorus exists in different allotropic forms like red, white black and yellow.Black phosphorus is the most thermodynamically stable allotropic form of phosphorus. It is stable at room temperature and pressure.

There are two types of black phosphorus as follows:
1) ${{\alpha }}$-black phosphorus: ${{\alpha }}$-black phosphorus is synthesized by heating of red phosphorus in a sealed tube at $803{{ K}}$. ${{\alpha }}$-black phosphorus does not oxidise in air.
2) ${{\beta }}$-black phosphorus: ${{\beta }}$-black phosphorus is synthesized by heating of white phosphorus under high pressure at $473{{ K}}$.

Black phosphorus has an orthorhombic structure. It resembles the structure of a honeycomb. The lattice of black phosphorus is an interlinked six-membered ring in which each atom is bonded to three other atoms. Thus, the reactivity of black phosphorus is very less.

Being most stable, black phosphorus is less reactive in nature. Thus, the most thermodynamically stable allotropic form of phosphorus is black.

Thus, the correct option is (C) black.

Note: The structure of black phosphorus is similar to that of graphite. Black phosphorus is black and flaky and a good conductor of electricity. Black phosphorus has puckered sheets of linked atoms. Electrons in layered black phosphorus behave in anisotropic manner.