The correct order of ionic radii of Y³⁺, La³⁺, Eu³⁺ and Lu³⁺ is
(Atomic nos. Y = 39, La = 57, Eu = 63, Lu = 71)

The basic character of the transition metal monoxides follows the order
(Atomic no's. Ti = 22, V = 23, Cr = 24, Fe = 26)

Which one of the following characteristics of the transition metals is associated with their catalytic activity?

Zn gives H₂ gas with H₂SO₄ and HCl but not with HNO₃ because