Question

What is the oxidation number of phosphorus?

Answer 1

Hint : Let’s first discuss what oxidation number is. The oxidation number, sometimes referred to as oxidation state, describes the degree of oxidation of an atom in a chemical compound. The oxidation state, which may be positive, negative or zero, is the hypothetical charge that an atom would have if all bonds to atoms of different elements were ionic, with no covalent component.

Complete Step By Step Answer:
The oxidation number of a free element is always zero. The oxidation number of a monatomic ion equals the charge of the ion. The oxidation number of hydrogen is $+ 1$ , but it is $- 1$ in when combined with less electronegative elements. The oxidation number of oxygen in compounds is usually $- 2$ , but it is $- 1$ in peroxides.
Phosphorus is generally considered to bear a $+ 5$ oxidation state, but several lower redox states have been reported, including the toxic gas phosphine. Phosphorus belongs to group $15$ . It has five electrons in its valence shell. So it can either lose $5$ electrons to attain $+ 5$ oxidation state or gain three electrons to attain a $- 3$ oxidation state. Hence its oxidation state varies from $- 3$ in $P{H_3}$ to $+ 5$ in ${H_3}P{O_4}$
Phosphorus commonly exhibits oxidation states of $- 3$ with active metals and of $+ 3$ and $+ 5$ with more electronegative nonmetals. The halogens and oxygen will oxidize phosphorus. The oxides are phosphorus oxide, and phosphorus $(III)$ oxide.

Note :
Lowest oxidation number is the least increase in oxidation no.an element can undergo. The least amount of electrons lost by an element to increase its oxidation no. and vice versa. For instance Nitrogen's lowest oxidation number is $- 3$ in $N{H_3}$ for instance.