Animated Solution for Chemistry - s and p-Block Elements: The order of the oxidation state of the phosphorus atom in ${\text{H}}_3{\text{PO}}_2$, ${\text{H}}_3{\text{PO}}_4$, ${\text{H}}_3{\text{PO}}_3$ and ${\text{H}}_4{\text{P}}_2{\text{O}}_6$ is

Imagine you are a molecular architect. You have a central phosphorus atom, and you start attaching oxygen atoms and hydroxyl groups to it. Depending on how many you attach, you create a whole family of compounds known as the oxoacids of phosphorus.

In this problem, we are tasked with finding the oxidation state of the central phosphorus atom in four different oxoacids: ${\text{H}}_3{\text{PO}}_4$, ${\text{H}}_4{\text{P}}_2{\text{O}}_6$, ${\text{H}}_3{\text{PO}}_3$, and ${\text{H}}_3{\text{PO}}_2$. The oxidation state is essentially a measure of how electron-poor or electron-rich an atom is within a molecule. Since oxygen is highly electronegative, it pulls electron density away from phosphorus, increasing its oxidation state.

To find the oxidation state, we don't necessarily need to draw the complex 3D structures of these molecules. We can use a highly reliable algebraic shortcut.

The rule is simple: For any neutral molecule, the algebraic sum of the oxidation states of all its constituent atoms must be exactly zero.

We also rely on two standard conventions: 1. Hydrogen, when bonded to non-metals, almost always has an oxidation state of $+1$. 2. Oxygen, except in rare cases like peroxides, has an oxidation state of $-2$.

Let's denote the unknown oxidation state of phosphorus as $x$. Now, we can systematically conquer each molecule.

1. Orthophosphoric Acid (${\text{H}}_3{\text{PO}}_4$) We have 3 hydrogens, 1 phosphorus, and 4 oxygens. Setting up our equation:

Here, phosphorus is in its maximum possible oxidation state of $+5$.

2. Hypophosphoric Acid (${\text{H}}_4{\text{P}}_2{\text{O}}_6$) Don't let the two phosphorus atoms trick you! We just adjust our equation to account for both of them:

Each phosphorus atom sits comfortably at a $+4$ oxidation state.

3. Phosphorous Acid (${\text{H}}_3{\text{PO}}_3$) Applying the same logic:

The oxidation state drops to $+3$.

4. Hypophosphorous Acid (${\text{H}}_3{\text{PO}}_2$) Finally, for the most oxygen-deprived acid in our list:

Phosphorus is at a mere $+1$ oxidation state here.

We have successfully extracted the oxidation states: $+5$, $+4$, $+3$, and $+1$.

Arranging them in strictly decreasing order, we get:

This perfectly matches option (A). While the algebraic method is incredibly fast for exams, remembering the structures of these oxoacids is equally important, as it reveals the physical reality behind these numbers—specifically, how many P-O and P-H bonds exist in the molecule!