Answer the question on the basis of passage given below:
In the periodic table, the d-block contains the elements of group 3 to 12. The d-orbitals are progressively filled in each of the four long periods. The elements of d-block referred as transition metals have partly filled d-orbitals and exhibit certain characteristic properties such as variety of oxidation states, formation of coloured ions, act as catalyst and show paramagnetic behaviour.
The two inner transition metals series 4f and 5f are known as Lanthanoids and Actinoids respectively. The lanthanoids resemble one another more closely as compared to ordinary transition elements in any series.
The catalytic activity of transition metals and their compounds is ascribed mainly to
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → their ability to adopt variable oxidation states.
The catalytic activity of transition metals is due mainly to their ability to adopt multiple oxidation states and to form complexes. A metal ion can take electrons from one reactant and give them to another by changing its oxidation state, opening an easier path.
Example: \( \ce{Fe^{3+}} \) catalyses the reaction between iodide and persulphate by shuttling between +3 and +2:
\( \ce{2Fe^3+ + 2I^- -> 2Fe^2+ + I2} \)
\( \ce{2Fe^2+ + S2O8^2- -> 2Fe^3+ + 2SO4^2-} \)
Why the other options are incorrect
Option (2) their chemical reactivity. — Incorrect. Transition metals are, in general, not very reactive (many are noble); high reactivity would use up the catalyst, not regenerate it.
Option (3) their magnetic behaviour. — Incorrect. Paramagnetism comes from unpaired electrons; it explains attraction to a magnet, not catalytic activity.
Option (4) their outer most orbital which has two electrons. — Incorrect. Most of these metals do have an ns2 configuration, but so do the alkaline earth metals, which are not good catalysts; it is the incomplete d subshell that matters.
NCERT (2026-27 edition):
Catalytic Properties
