Question

How does phosphorus form phosphorus pentoxide? it's valency is 3 right? So, it should be P2O3?

Answer 1

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P2O5 (or P4O10) can be prepared by burning elemental P4 (tetra-phosphorus) in an environment containing a sufficient amount of oxygen. The chemical reaction for the preparation of P2O5 is provided below.

${\huge{}}$${\huge{}}$P4 + 5O2 → P4O10

This compound is a source of pure phosphoric acid.