The oxidation number of phosphorus is +3. In compounds of phosphorus, the most common oxidation numbers of phosphorus are 5, 3, and -3. The oxidation number is also referred to as the oxidation state.
The oxidation number of an element in a compound is the number of electrons gained or lost by an atom that forms the compound. It is expressed as a positive or negative number that shows the ionic charge of the element.
In electrochemical reactions, oxidation numbers are used to identify the number of electrons lost or gained by each atom in the reaction. These oxidation numbers are assigned to compounds using a number of rules. Such as, the oxidation number of a free element is always zero; the oxidation number of hydrogen is +1; the oxidation number of a monatomic ion equals the charge of the ion and the oxidation number of oxygen in a compound is -2.