The transition elements have incompletely filled d-subshells in their ground state or in any of their oxidation states. The transition elements occupy position in between s- and p-blocks in groups 3-12 of the Periodic table. Starting from the fourth period, the transition elements form four series: Sc to Zn, Y to Cd, La and Hf to Hg, and Ac and Rf to Cn. In general, the electronic configuration of outer orbitals of these elements is (n - 1) d1-10 ns1-2. The electronic configurations of outer orbitals of Zn, Cd, Hg and Cn are represented by the general formula (n - 1)d10 ns2. All the transition elements have typical metallic properties such as high tensile strength, ductility, malleability. Except mercury, which is liquid at room temperature, other transition elements have typical metallic structures. The transition metals and their compounds also exhibit catalytic property and paramagnetic behaviour. Transition metals also form alloys. An alloy is a blend of metals prepared by mixing the components. Alloys may be homogeneous solid solutions in which the atoms of one metal are distributed randomly among the atoms of the other.
Which of the following characteristics of transition metals is associated with higher catalytic activity?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Variable oxidation states
The passage says that transition metals and their compounds show catalytic property, but not why. Their catalytic activity comes mainly from their ability to adopt multiple oxidation states (and to form complexes).
A metal ion that can switch easily between two oxidation states can take an electron from one reactant and pass it to another, giving the reaction a path of lower activation energy.
Iron(III) catalyses the reaction between iodide and persulphate ions in this way:
\( \displaystyle \ce{2I^- + S2O8^2- -> I2 + 2SO4^2-} \)
\( \displaystyle \ce{2Fe^3+ + 2I^- -> 2Fe^2+ + I2} \)
\( \displaystyle \ce{2Fe^2+ + S2O8^2- -> 2Fe^3+ + 2SO4^2-} \)
\( \ce{Fe^3+} \) is reduced to \( \ce{Fe^2+} \) and then oxidised back, so it is regenerated at the end, as a catalyst must be.
Why the other options are incorrect
Option (1) High enthalpy of atomisation — Incorrect. It shows strong metallic bonding (many unpaired d electrons) and goes with high melting and boiling points, not with catalysis.
Option (3) Paramagnetic behaviour — Incorrect. Paramagnetism is a magnetic property caused by unpaired electrons; it does not provide a reaction path.
Option (4) Colour of hydrated ions — Incorrect. The colour comes from d–d transitions, in which visible light excites a d electron to a higher d level; it has nothing to do with catalysis.
NCERT (2026-27 edition):
4.3.12 Catalytic Properties – “The transition metals and their compounds are known for their catalytic activity. This activity is ascribed to their ability to adopt multiple oxidation states and to form complexes. … This has the effect of increasing the concentration of the reactants at the catalyst surface and also weakening of the bonds in the reacting molecules (the activation energy is lowering). Also because the transition metal ions can change their oxidation states, they become more effective as catalysts. For example, iron(III) catalyses the reaction between iodide and persulphate ions.”