In the form of dichromate, Cr(VI) is a strong oxidising agent in acidic medium but Mo(VI) in `Mo0_(3)` and W(VI) in `W0_(3)` are not because

Options

A. Cr (VI) is more stable than Mo(VI) and W(VI).
B. Mo (VI) and W (VI) are more stable than Cr(VI).
C. Higher oxidation states of heavier members of group-6 of transition series are more stable.
D. Lower oxidation states of heavier members of group-6 of transition series are more satble.

Explanation

To answer the question, we need to analyze the oxidation states of chromium, molybdenum, and tungsten, and understand why Cr(VI) acts as a strong oxidizing agent while Mo(VI) and W(VI) do not.

### Step-by-Step Solution:

1. **Understanding Oxidizing Agents**:
- An oxidizing agent is a substance that gains electrons in a chemical reaction and is reduced in the process. Strong oxidizing agents are typically less stable in their higher oxidation states.

2. **Oxidation States of Chromium, Molybdenum, and Tungsten**:
- Chromium (Cr) in the form of dichromate (Cr2O7^2-) is in the +6 oxidation state (Cr(VI)).
- Molybdenum (Mo) in MoO3 is in the +6 oxidation state (Mo(VI)).
- Tungsten (W) in WO3 is also in the +6 oxidation state (W(VI)).

3. **Stability of Higher Oxidation States**:
- In the case of chromium, the +6 oxidation state is less stable compared to the +6 oxidation states of molybdenum and tungsten. This is due to the fact that chromium is a lighter element and has a higher tendency to be reduced.

4. **Heavier Elements and Stability**:
- Molybdenum and tungsten, being heavier elements, have their +6 oxidation states more stabilized due to their electronic configurations. The energy difference between the s and d orbitals is smaller in heavier elements, allowing them to maintain their higher oxidation states more effectively.

5. **Conclusion**:
- Therefore, Cr(VI) is a strong oxidizing agent because it is less stable in its +6 state compared to Mo(VI) and W(VI), which are more stable and do not act as oxidizing agents.

### Final Answer:
Cr(VI) is a strong oxidizing agent in acidic medium because it is less stable than Mo(VI) and W(VI), which are more stable in their higher oxidation states.