The three important oxidation states of Phosphorus are:

Concept:

• Phosphorus is a group 15 or group V A element.
• It has outer electronic configuration ns²np³.
• Phosphorus unlike Nitrogen exists in many forms such as red phosphorus, white phosphorus, etc, each of them having a different lattice structure.
• Phosphorus mainly exists as P₄, and is a tetrahedral molecule where each Phosphorus atom is linked to another P atom via single covalent bonds.
• This makes Phosphorus to be much more reactive than Nitrogen as single bonds are much easier to break.
• The breaking of the P-P bond requires much less energy.
• This makes Phosphorus quite reactive in nature.

Explanation:

• The element P has five electrons in the outermost shell and is short of three electrons to complete its octet.
• One case may be such that P can accept three electrons from other elements and be anion P^3-, which is not easy considering the energy of electron gain enthalpy.
• So, they make mostly covalent compounds by sharing three electrons in most cases and P^3- is formed in very rare cases.
• As Phosphorus has a vacant 3d orbital, which can accept electrons in it, it can expand its octet and sometimes exhibits a covalency of 5 in compounds such as PCl₅.
• In PCl₅, the oxidation state of P is +5, in that case, all 5 valence electrons take part in bonding.
• In PCl₃, the oxidation state of P is +3.
• In P₂O₅ also, the oxidation state of P is +5.
• Phosphorus shows the oxidation state of -3 in compounds such as phosphine, PH₃.

Hence, the three important oxidation states of Phosphorus are -3, +3 and +5.