Question

The oxidation state of phosphorus is maximum in_____
A.Phosphine $P{H_3}$
B.Diphospine ${P_2}{H_4}$
C.Metaphosphoric acid $HP{O_3}$
D.Phosphorus acid ${H_3}P{O_3}$

Answer 1

Hint: We need to remember that unlike other elements phosphorus has a distinct property when it comes to oxidation state. We know that it is an element with atomic number $15$ and it has a valency of $5$. This element when forms a compound with other elements it can form compounds, bonding with different valence states.

Complete step by step answer:
We know that Phosphorus has a valency of number $5$. But it has a distinct property of extended valencies. Having a valency number of $5$ means it can either lose $5$ electrons or either gain $3$ electrons to satisfy its valency (to complete an octet). Hence its valency ranges from $ - 3$ to $ + 5$.
Let us consider phosphine $P{H_3}$ .
In this chemical formula, one phosphorus atom is bonded with $3$ hydrogen atoms. We know that hydrogen atoms have an oxidation number of $ + 1$. Thus for $3$ hydrogen atoms the oxidation state is $ + 3$. To satisfy this oxidation state the phosphorus exhibits oxidation of $ - 3$, that is it accepts $3$ electrons from hydrogen. Hence, in phosphine the oxidation number of phosphorus is $ - 3$.
Let us consider diphospine ${P_2}{H_4}$.
In this chemical formula there are two phosphorus atoms bonded with four hydrogen atoms. The oxidation state of hydrogen is $ + 1$ , hence for 4 hydrogen atoms it is $ + 4$ . Now there are two phosphorus atoms in bonding, thus two atoms will share $ - 4$ valency equally as -2 oxidation number for each phosphorus to satisfy the valency of hydrogen atoms. Thus, in diphospine the oxidation number of phosphorus is $ - 2$ .
Let us consider metaphosphoric acid $HP{O_3}$
In this chemical formula, one phosphorus atom is bonded with one hydrogen and three oxygen atoms. We know the oxidation state of hydrogen is $ + 1$ and that of oxygen is $ - 2$. Thus, for hydrogen oxidation is 1 and for three oxygen atoms oxidation state is $ - 6$. Thus $1 - 6 = - 5$ . Now one phosphorus atom should satisfy this valency, hence the phosphorus will exhibit us considering the oxidation state of $ + 5$.
Let us consider phosphorus acid ${H_3}P{O_3}$.
This chemical formula has one phosphorus atom bonded with three hydrogen and three oxygen atoms. Hence, the oxidation state of hydrogen is $ + 3$ and that of oxygen is $ - 6$, in total it is $ - 3$. Hence to satisfy this valency the phosphorus atom will exhibit oxidation state of $ + 3$.
Hence, from the above evaluation we can see that the highest state of phosphorus is $ + 5$ which is found in metaphosphoric acid $HP{O_3}$ .
Hence, the correct answer to the question is option C.

Note:
We must have to remember that phosphorus is an element which doesn’t exist in a free state naturally in earth’s atmosphere because it is highly reactive. It is found in two different types – one is red phosphorus and another is white phosphorus. Phosphorus is essential for life as it is a constituent of DNA, RNA and ATP in living bodies. It is also used in industries in detergents and pesticides. It forms bonds with other elements easily since it is highly reactive and also because it has a distinct property of extended oxidation state from $ - 3$ to $ + 5$.