Asserton :- The formal oxidation no. of sulphur in `Na_(2)S_(4)O_(6)` is `2.5`
Reason :- Two S-atoms are not directly linked with O-atoms.

Options

A. If both Assertion & Reason are True & the Reason is a correct explanation of the Assertion.
B. If both Assertion & Reason are True but Reason is not a correct explanation of the Assertion.
C. If Assertion is True but the Reason is False.
D. If both Assertion & Reason are False.

Answer

To determine the formal oxidation number of sulfur in the compound \( \text{Na}_2\text{S}_4\text{O}_6 \) and evaluate the assertion and reason provided, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Components of the Compound:**
- The compound \( \text{Na}_2\text{S}_4\text{O}_6 \) consists of sodium (Na), sulfur (S), and oxygen (O).

2. **Assign Known Oxidation States:**
- Sodium (Na) typically has an oxidation state of +1.
- Oxygen (O) typically has an oxidation state of -2.

3. **Set Up the Equation for Oxidation States:**
- Let the oxidation state of sulfur be \( x \).
- There are 2 sodium atoms contributing \( 2 \times (+1) = +2 \).
- There are 6 oxygen atoms contributing \( 6 \times (-2) = -12 \).
- There are 4 sulfur atoms contributing \( 4x \).

4. **Write the Overall Charge Equation:**
- Since the compound is neutral, the sum of oxidation states must equal 0:
\[ 4x + 2 - 12 = 0 \]

5. **Solve for \( x \):**
- Rearranging the equation gives:
\[ 4x - 10 = 0 \implies 4x = 10 \implies x = \frac{10}{4} = 2.5 \]

6. **Conclusion on the Assertion:**
- The assertion that the formal oxidation number of sulfur in \( \text{Na}_2\text{S}_4\text{O}_6 \) is 2.5 is correct.

7. **Evaluate the Reason:**
- The reason given is that two sulfur atoms are not directly linked with oxygen atoms. In the structure of \( \text{Na}_2\text{S}_4\text{O}_6 \), two sulfur atoms are indeed bonded to each other rather than being bonded to oxygen. This supports the assertion as it indicates that the oxidation state of those sulfur atoms is not influenced by oxygen.

8. **Final Conclusion:**
- Both the assertion and reason are correct, and the reason correctly explains the assertion.

### Final Answer:
- The assertion is correct, and the reason is also correct. Therefore, the answer is option A.