Identify the correct order of decreasing number of `pi` bonds in the structures of the following molecules
(I) `H_2S_2O_6`
(II) `H_2SO_3`
(III) `H_2S_2O_5`

Options

A. `I gt IIgt III`
B. `II gt IIIgt I`
C. `I gt IIIgt II`
D. `II gt I gt III`

Answer

To solve the problem of identifying the correct order of decreasing number of π bonds in the structures of the given molecules (I) H₂S₂O₆, (II) H₂SO₃, and (III) H₂S₂O₅, we will follow these steps:

### Step 1: Draw the Structures of Each Molecule

1. **H₂S₂O₆ (Dithionic Acid)**:
- The structure consists of two sulfur atoms (S) connected to each other.
- Each sulfur atom is bonded to oxygen atoms, with some of these bonds being double bonds.
- The structure can be represented as:
```
O=S(OH)-S(=O)(OH)
```
- In this structure, there are 4 π bonds (two from each double bond).

2. **H₂SO₃ (Sulfurous Acid)**:
- The sulfur atom is bonded to three oxygen atoms, one of which is a hydroxyl group (OH).
- The structure can be represented as:
```
O=S(OH)-O
```
- In this structure, there is 1 π bond (from the double bond with oxygen).

3. **H₂S₂O₅ (Disulfurous Acid)**:
- The structure consists of two sulfur atoms connected to each other with multiple oxygen atoms.
- The structure can be represented as:
```
O=S(OH)-S(=O)-O
```
- In this structure, there are 3 π bonds (two from the double bonds and one from the OH group).

### Step 2: Count the Number of π Bonds in Each Structure

- **H₂S₂O₆**: 4 π bonds
- **H₂SO₃**: 1 π bond
- **H₂S₂O₅**: 3 π bonds

### Step 3: Arrange the Molecules in Order of Decreasing Number of π Bonds

Based on the counts:
- H₂S₂O₆ has the most π bonds (4).
- H₂S₂O₅ comes next with 3 π bonds.
- H₂SO₃ has the least with 1 π bond.

Thus, the order of decreasing number of π bonds is:
1. H₂S₂O₆ (4 π bonds)
2. H₂S₂O₅ (3 π bonds)
3. H₂SO₃ (1 π bond)

### Final Answer
The correct order of decreasing number of π bonds is:
**H₂S₂O₆ > H₂S₂O₅ > H₂SO₃**.