Which allotropic form of phosphorus is poisonous?

Phosphorus exists in several allotropic forms, including white, red, black, and violet phosphorus. Among these, white phosphorus (P₄) is the most reactive and toxic. It is composed of tetrahedral P₄ molecules, in which four phosphorus atoms are linked in a strained ring. The high ring strain and reactive P–P bonds make it highly unstable and chemically active.

White phosphorus is poisonous because it can be readily oxidized in air, producing phosphorus pentoxide (P₂O₅) and toxic phosphorus oxides, which are harmful if inhaled or ingested. Even small amounts can cause severe damage to internal organs, especially the liver and kidneys, and can lead to phossy jaw in chronic exposure. It also glows in the dark due to chemiluminescence, a phenomenon caused by slow oxidation.

In contrast:

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  Red phosphorus is more stable, non-volatile, and relatively non-toxic. It is used in safety matches and fireworks.

  
  


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  Black phosphorus is the most stable form, chemically inert, and exhibits semiconducting properties, making it safe compared to white phosphorus.

  
  


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  Violet phosphorus (Hittorf’s phosphorus) is intermediate in stability and also much less toxic.

  
  

Because of its extreme toxicity, white phosphorus must be stored underwater or in inert atmospheres to prevent accidental ignition and poisoning. Its highly reactive nature, combined with the formation of toxic oxides, makes it the only poisonous allotropic form of phosphorus widely recognized in chemistry.

Therefore, the correct answer is White phosphorus (P₄), highlighting its dangerous reactivity and toxic properties.