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

The electronic configuration of cerium ($Z=58$) is

$$[Xe],4f^1,5d^1,6s^2$$

• Option (A)

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

  Cerium readily forms $$Ce^{3+}$$ and $$Ce^{4+}$$ ions. The $$Ce^{4+}$$ ion attains the stable noble gas configuration $$[Xe]$$.

  Hence, statement (A) is correct.

• Option (B)

  Cerium(IV) acts as an oxidizing agent.

  Since the $$+3$$ oxidation state is more stable, $$Ce^{4+}$$ readily gains an electron to form $$Ce^{3+}$$.

  $$Ce^{4+}+e^- \rightarrow Ce^{3+}$$

  Therefore, $$Ce^{4+}$$ acts as an oxidizing agent.

  Hence, statement (B) is correct.

• Option (C)

  The $$+4$$ oxidation state of cerium is not known in solutions.

  This statement is incorrect. Cerium(IV) compounds such as ceric ammonium nitrate (CAN) and ceric sulfate are stable in aqueous solutions and are widely used in analytical chemistry.

  Hence, statement (C) is incorrect.

• Option (D)

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

  For lanthanides, the $$+3$$ oxidation state is the most stable and predominant oxidation state.

  Hence, statement (D) is correct.

Therefore, the incorrect statement is option C.