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(i) Transition elements show variable oxidation states in their compounds. This is because there is a very small energy difference or almost the same energy difference in between ‘(n-1)d’ and ‘ns’ orbitals. Due to which both electrons of (n-1)d orbitals as well as ns-orbitals can take part in bond formation. So, variable valencies are seen.

(ii) We have studied that as we go down a group or towards the bottom of the periodic table, we would expect those elements to be heavier than those at the top. But, due to lanthanide contraction, Zr (At. No. 40) and Hf (At. No. 72) have almost identical radii.

The lanthanide contraction is the ‘greater than expected decrease’ in ionic radii of the elements in the lanthanide series from atomic number 57 - lanthanum, to 71 - lutetium, which results in smaller than otherwise expected ionic radii for the subsequent elements. This is because atomic radii of 4d and 5d transition elements are nearly same and this similarity in size is due to weak shielding of d-electrons.

(iii) We know, transition elements show variable oxidation states in their compounds as there is a very small energy difference or almost the same energy difference in between ‘(n-1)d’ and ‘ns’ orbitals.

Transition metals and their compounds function as catalysts because of their ability to change oxidation states. Also, they can adsorb other substances on to their surface and activate them in the process.

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(ii)

(iii)