Which one the following statement regarding helium is incorrect?

Excess of KI reacts with CuSO₄ solution and then Na₂S₂O₃ solution is added to it. Which of the statements is incorrect for this reaction?

Aluminium chloride exists as dimer, Al₂Cl₆ in solid state as well as in solution of non-polar solvents such as benzene. When dissolved in water, it gives :

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