Cerium (Z = 58) is an important member of the lanthanoids. Which of the following statements about cerium is incorrect?

The +4 oxidation state of cerium is not known in solution

The correct answer is option 3. The +4 oxidation state of cerium is not known in solution.

Let us delve into each statement about cerium (Ce) to understand its oxidation states and properties:

1. The common oxidation states of cerium are +3 and +4.

Cerium is known to exhibit two common oxidation states: +3 and +4. In its +3 oxidation state \((Ce^{3+})\), cerium forms stable compounds where it behaves as a typical lanthanoid (lanthanide) ion with a filled or partially filled 4f shell. In its +4 oxidation state \((Ce^{4+})\), cerium loses one more electron compared to \(Ce^{3+}\), resulting in a \(4f^0\) configuration, which is less common but stable under certain conditions.

2. The +3 oxidation state of cerium is more stable than +4 oxidation state.

This statement is generally true. In aqueous solutions and in most chemical environments, cerium prefers to exist in the +3 oxidation state \((Ce^{3+})\) because it is more stable. \(Ce^{3+}\) is commonly found in cerium compounds and complexes due to its more favorable electronic configuration involving partially filled 4f orbitals.

3. The +4 oxidation state of cerium is not known in solution.

This statement is incorrect. Cerium in the +4 oxidation state \((Ce^{4+})\) is indeed known and can exist in solution under suitable conditions, particularly in acidic solutions where \(Ce^{4+}\) forms stable complexes. \(Ce^{4+}\) is a strong oxidizing agent and can readily accept electrons to form \(Ce^{3+}\).

4. Cerium (IV) acts as an oxidizing agent.

This statement is correct. \(Ce^{4+}\) is a powerful oxidizing agent due to its ability to accept electrons. It is used in various redox reactions where it can oxidize other species, converting \(Ce^{4+}\) to \(Ce^{3+}\) in the process.

Summary:

Cerium commonly exists in the +3 and +4 oxidation states.

\(Ce^{3+}\) is more stable than Ce^4+ in most environments.

\(Ce^{4+}\) is known and stable in solution, especially in acidic conditions.

\(Ce^{4+}\) acts as a strong oxidizing agent in chemical reactions.

Therefore, the incorrect statement provided in the options was: The +4 oxidation state of cerium is not known in solution.

This statement is incorrect because cerium in the +4 oxidation state \((Ce^{4+})\) is indeed known and stable in solution under appropriate conditions.