How would you account for the following?
(l) Transition metals exhibit variable oxidation states.
(ii) Zr (Z = 40) and Hf (Z = 72) have almost identical radii.
(iii) Transition metals and their compounds act as catalyst.

Explanation

(i) The variable oxidation states of transition elements are due to the participation of ns and ( n -1) d-electrons in bonding. Lower oxidation state is exhibited when ns-electrons take part in bonding. Higher oxidation states are exhibited when ( n - 1) d - electrons take part in bonding.
(ii) This is because the atomic radii of 4 d and 5d transition elements are nearly same. This similarity in size is consequences of Lanthanide contraction. Because of this lanthanide contraction the radii of Hf becomes nearly equal to that of Zr.
(iii) Transition elements act as good catalyst in chemical reaction because they can lend electrons or withdraw electrons from the reagent, depending on the nature of the reaction.
The ability of transition metals to be in a variety of oxidation states, the ability to interchange ·between the.oxidation states and the ab1hty to form complexes withthe reagents and be a good source for electrons make transition metals good catalysts. ·