(a) Account for the following:
(i) Transition metals show variable oxidation states.
(ii) Zn, Cd and Hg are soft metals.
(iii) E° value for the Mn³⁺/Mn²⁺ couple is highly positive (+ 1·57 V) as compared to Cr³⁺/Cr²⁺ . 56/1 12
(b) Write one similarity and one difference between the chemistry of lanthanoid and actinoid elements.

OR

(a) Following are the transition metal ions of 3d series:
Ti⁴⁺, V²⁺, Mn³⁺, Cr³⁺ (Atomic numbers: Ti = 22, V = 23, Mn = 25, Cr = 24) Answer the following:
(i) Which ion is most stable in an aqueous solution and why?
(ii) Which ion is a strong oxidising agent and why?
(iii) Which ion is colourless and why?
(b) Complete the following equations:
(i) $2{\mathrm{MnO}}_4^{-}+16{\mathrm{H}}^{+}+5{\mathrm{S}}^{2-}⟶$
(ii) ${\mathrm{KMnO}}_4\stackrel{\text{ heat }}{⟶}$

(a) (i) ns-electrons, (n-1) d electron has similar energy. They both can be removed easily. Therefore, transition elements show variable oxidation state.
(ii) These elements have completely filled d orbital due to which they form weak metal-metal bonds. Therefore, Zn, Cd, and Hg are soft metals.
(iii) When Mn³+ is converted to Mn²⁺ it attains the stable half-filled d5 configuration. Which results in large E0 value. While Cr³⁺ (d³) when converted into Cr ²⁺(d²) it goes to less stable configuration.
(b) The atomic radius of both lanthanoids and actinoids decreases with increase in atomic number.
All the Lanthanoids are non-radioactive except promethium while all the actinoids are radioactive.

OR

(a)
(i) V²⁺ and Cr³⁺ are most stable as they both have the [Ar]3d3 or 𝑡³_2g stable electronic configuration.
(ii) Mn³⁺ ions are the strongest oxidising agent in the aqueous solution because after converting to Mn²⁺ they get stable half-filled d⁵ electronic configuration.
(iii) Ti⁴⁺ has no electron in d orbital. No d-d transition is possible in this case. Therefore, it is colourless.
(b)
(i) $2{\mathrm{MnO}}_4^{-}+16{\mathrm{H}}^{+}+5{\mathrm{S}}^{2-}\to 2{\mathrm{Mn}}^{2+}+8{\mathrm{H}}_2\mathrm{O}+5\mathrm{S}$
(ii) $2{\mathrm{KMnO}}_4\stackrel{\text{ heat }}{⟶}{\mathrm{K}}_2{\mathrm{MnO}}_4+{\mathrm{MnO}}_2+{\mathrm{O}}_2$