What is the allotrope of phosphorus in which P-atom completes its octet?

White phosphorus

The correct answer is 3. White phosphorus.

In white phosphorus, each phosphorus atom is bonded to three other phosphorus atoms, forming a tetrahedral P4 molecule. This arrangement allows each phosphorus atom to share its valence electrons with three other phosphorus atoms, effectively completing its octet.

Here's a more detailed explanation of the allotropes of phosphorus:

• White phosphorus: This is the most reactive allotrope of phosphorus and exists as a waxy, solid tetrahedral P4 molecule.
• Red phosphorus: This is a less reactive allotrope of phosphorus and exists as a red, amorphous solid. It is formed by heating white phosphorus.
• Alpha-black phosphorus: This is a highly reactive allotrope of phosphorus and exists as a black, metallic solid. It is formed by subjecting red phosphorus to high pressure.
• Beta-black phosphorus: This is a less reactive allotrope of phosphorus and exists as a black, layered solid. It is formed by heating alpha-black phosphorus.

So, among these allotropes, only white phosphorus has a structure that allows each phosphorus atom to complete its octet, satisfying the octet rule.