Question

How would you account for the following?
A) Transition metals exhibit variable oxidation states.
B) ${\text{Zr}}$ $\left( {{\text{Z}} = 40} \right)$ and ${\text{Hf}}$ $\left( {{\text{Z}} = 72} \right)$ have almost identical radii.
C) Transition metals and their compounds act as catalysts.

Answer 1

The elements in the d-block of the periodic table are known as the transition elements or transition metals. The d-block occupies the central part of the periodic table from group-3 to group-12.

Complete answer:
A) A number assigned to an element in a chemical formula that represents the number of electrons gained or lost by that element is known as the oxidation number.
The valence shell electronic configuration of transition metals is $n{s^2}\left( {n - 1} \right)d$. Both the electrons of ns shell and the electrons of $\left( {n - 1} \right)d$ shell participate in the bonding.
When the electrons of the ns shell participate in bonding, the transition metals exhibit a lower oxidation state.
When the electrons of the $\left( {n - 1} \right)d$ shell participate in bonding, the transition metals exhibit a higher oxidation state.
Thus, transition metals exhibit variable oxidation states.
B) ${\text{Zr}}$ belongs to the 4d transition series of the periodic table and ${\text{Hf}}$ belongs to the 5d transition series of the periodic table.
But both ${\text{Zr}}$ and ${\text{Hf}}$ show similar physical and chemical properties due to the lanthanide contraction.
A steady decrease in the atomic sizes without increasing the atomic number from the element lanthanum $\left( {{\text{Z}} = 57} \right)$ through lutetium $\left( {{\text{Z}} = 71} \right)$ is known as lanthanide contraction.
Thus, ${\text{Zr}}$ $\left( {{\text{Z}} = 40} \right)$ and ${\text{Hf}}$ $\left( {{\text{Z}} = 72} \right)$ have almost identical radii due to lanthanide contraction.
C) Depending on the nature of the reaction, the transition metals can withdraw or donate electrons from the reagents.
Transition metals exhibit variable oxidation states, also they have an ability to interchange between the oxidation states and can form complexes with the reagents.
Due to these reasons, transition metals act as a good source of electrons.
Thus, the transition metals and their compounds act as catalysts.

Note: Transition metals generally have partially filled d-orbitals. Examples of transition metals are: scandium, iron, nickel, cobalt, titanium, etc. These elements form coloured compounds and ions by d-d transition of electrons.