Question

The average oxidation state of sulphur in the tetrathionate ion is:

• A. +3
• B. +2.5
• C. +5
• D. +3.5
• E. +1.5

Hint:

Fractional oxidation states often indicate that the element exists in different bonding environments within the same molecule.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Oxidation state is the formal charge assigned to an atom. The average oxidation state in a polyatomic ion is found using the overall charge.
Step 2: Key Formula or Approach:
The formula for tetrathionate ion is $\text{S}_4\text{O}_6^{2-}$.
Sum of oxidation states = Net charge.
Step 3: Detailed Explanation:
Let the oxidation state of sulphur be $x$.
Oxidation state of oxygen is typically $-2$.
For $\text{S}_4\text{O}_6^{2-}$: \[ 4(x) + 6(-2) = -2 \] \[ 4x - 12 = -2 \] \[ 4x = 10 \] \[ x = \frac{10}{4} = +2.5 \] Structurally, two S atoms have 0 and two have +5 oxidation states, averaging to +2.5.
Step 4: Final Answer:
The average oxidation state is +2.5.