Animated Solution for Chemistry - s and p-Block Elements: Number of $\text{Cl}=\text{O}$ bonds in chlorous acid, chloric acid and perchloric acid respectively are

Imagine you are a central chlorine atom. You have a bunch of valence electrons and empty d-orbitals just waiting to be used. When oxygen atoms come around, they are hungry for electrons. How do you satisfy them? You form oxyacids!

In this problem, we are looking at three specific oxyacids of chlorine: chlorous acid (${\text{HClO}}_2$), chloric acid (${\text{HClO}}_3$), and perchloric acid (${\text{HClO}}_4$). The question asks us a very simple structural detail: how many $\text{Cl}=\text{O}$ double bonds are present in each of these molecules?

Before we draw anything, let's establish a fundamental rule for the oxyacids of halogens. They are all monobasic. What does that mean? It means they only have one acidic hydrogen atom that they can donate. In the molecular structure, this acidic hydrogen is always bonded to an oxygen atom, forming a single $-\text{OH}$ (hydroxyl) group attached to the central halogen.

So, for any oxyacid of chlorine with the formula ${\text{HClO}}_n$, exactly one oxygen atom is used up to form the $-\text{OH}$ group. What happens to the remaining $(n-1)$ oxygen atoms? They form double bonds ($\text{Cl}=\text{O}$) with the central chlorine atom! Chlorine can do this because it is a period 3 element and can expand its octet using its empty d-orbitals, forming strong $p\pi -d\pi$ bonds.

Let's apply our golden rule to the three acids in the question.

1. Chlorous Acid (${\text{HClO}}_2$) We start with the central chlorine atom. We attach one $-\text{OH}$ group. We have one oxygen atom left. This remaining oxygen forms a double bond with chlorine. Therefore, the number of $\text{Cl}=\text{O}$ bonds in chlorous acid is 1.

2. Chloric Acid (${\text{HClO}}_3$) Again, chlorine is in the center. We attach our mandatory $-\text{OH}$ group. Now, we have two oxygen atoms left over. Both of these will form double bonds with the central chlorine. Therefore, the number of $\text{Cl}=\text{O}$ bonds in chloric acid is 2.

3. Perchloric Acid (${\text{HClO}}_4$) You probably see the pattern now! Central chlorine, one $-\text{OH}$ group. This leaves us with three oxygen atoms. All three of them form double bonds with chlorine. Therefore, the number of $\text{Cl}=\text{O}$ bonds in perchloric acid is 3.

Putting it all together, the number of $\text{Cl}=\text{O}$ bonds in chlorous acid, chloric acid, and perchloric acid are 1, 2, and 3, respectively.

This perfectly matches option (d). By simply understanding the basic structural framework of oxyacids—that there is always exactly one $-\text{OH}$ group—you can instantly deduce the number of double bonds just by looking at the chemical formula. No complex memorization required, just pure chemical logic!