The Lanthanoids

Let’s evaluate each option:
1. Option (A): - The green manganate ion is paramagnetic, not diamagnetic. The permanganate ion is also paramagnetic, not diamagnetic. Hence, this statement is incorrect.
2. Option (B): - Interstitial compounds of transition metals do tend to have higher melting points than pure transition metals due to the presence of extra atoms that enhance the metallic bonding. Their compounds are usually less reactive. Thus, this statement is incorrect.
3. Option (C): - Cerium (Ce) is a lanthanoid metal that exhibits a stable oxidation state of as well as the oxidation state of . Cerium is unique because it can stabilize both these oxidation states, making this statement correct.
4. Option (D): - Chromium (Cr) in the oxidation state is a strong oxidizing agent, while Tungsten (W) in the oxidation state is more stable.
Thus, this statement is incorrect. Therefore, the correct statement is (C): Cerium exists in both +4 and +3 oxidation states, with the +4 oxidation state being the most stable.