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Q.
Red phosphorus is chemically unreactive because
The p-Block Elements - Part2
Solution:
In red phosphorus, one of the $P _{4}$ bonds is broken, and one additional bond is formed with a neighbouring tetrahedron resulting in a more chain-like structure.
It is formed by heating white phosphorus to $250^{\circ} C$, it becomes an amorphous network of atoms that reduces strain and gives greater stability thereby renders it unreactive.