Question

White phosphorus is much more reactive than black or red phosphorus. Explain.

Short answer

Due to the presence of $P_4$molecules which are very unstable, white phosphorus is highly reactive than black or red phosphorus.

Step-by-step solution

Concept Introduction

Reactive chemicals are compounds that can give out heat, energy, or hazardous gases or vapours when they come into contact with air, water, or other common substances

Explanation for white phosphorus being more reactive than black or red phosphorus

The solid elements of Group$5A$exist as a number of allotropes.

White phosphorus is a waxy solid consisting of tetrahedral$P_4$molecules.

White phosphorus is very reactive and bursts into flame in air to yield${\text{P}}_{\text{4}}{\text{O}}_{\text{10}}$ .

These$P_4$ molecules are highly strained, and thus, an unstable arrangementwhich makes the white phosphorus extremely reactive.

Black phosphorus has a regular crystalline structure, but red phosphorusis amorphous and is thought to consist of chains of$P_4$units.

Red phosphorus can be obtained by heating white phosphorus at$\text{300°C}$in an inert atmosphere for several days. Unlike white phosphorus, redphosphorus does not ignite readily in air.

When phosphorus is heated under high pressure, a series of phases of blackphosphorus are formed, the thermodynamically most stable form.

Therefore, $P_4$molecules are unstable in white phosphorus