Mn²⁺ ⇒ 3d⁵ contains 5 unpaired electrons
n = 5,
\( \sqrt{n(n+ 2)} \) BM
\(= \sqrt{5(5+ 2)} = \sqrt{35} = 5.92 BM\)

Ce⁴⁺ gains electrons to form Ce³⁺ ion therefore it is considered a good oxidising agent

Cr(24): [Ar] 3d⁵4s¹

+3

(i) Transition metals have number of unpaired electron. They are involved in metallic bonding. Hence they show high melting point.
(ii) As we move from left to right along the transition metal series melting point first increases reach a maximum value and then decreases as the d-electrons pair up and become less available for bonding.

(i) The compounds of Cu²⁺ are coloured as it has one free electron its valence shell which absorb I radiation of visible region and get excited to emit its complementary colour.
(ii) Zn has no free electron it has fully filled shells. Due to extra stable orbitals electron can't be excited by radiations of visible light, hence its compounds are colourless.

In Gd⁺³ there are 64 electrons. Hence electronic configuration will be [Xe]4f⁷ 5d¹ 6s². Hence no electrons are there in outer d - orbital. Due to this it is colourless.

(i) (a) Vanadium
(b) Iron
(c) Silver
(ii) H₂S ⟶ 2H⁺+ S^2-
[MnO₄^- + 8H⁺ + 5e^- ⟶ Mn²⁺ + 4H₂O] x 2
[S2^- ⟶ S + 2e^-] x 5
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2MnO₄^- + 5S^2- + 16H⁺⟶ 2Mn²⁺ + 5S + 8H₂O
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(i) Mn³⁺/Mn²⁺ has large positive E^o value. Hence, Mn³⁺ can be easily reduced to Mn²⁺ because Mn²⁺ has half-filled electronic, configuration, so it is stable and Mn³⁺ is least stable. It is a good oxidising agent.

The steady decrease in the atomic/ionic radius from La³⁺ to Lu³⁺ is called lanthanoid contraction.

(i) In transition series, as we move down from Ti to Zn, the standard reduction potential E⁰_M²⁺/M³ value is approaching towards less negative value and copper has a positive reduction potential, i. e. elemental copper is more stable than Cu²⁺.
(ii) E⁰_M²⁺/M value for manganese and zinc are more negative than regular trend. It is due to extra stability arises due to the half filled d⁵ configuration in Mn²⁺ and completely filled d¹⁰ configuration in Zn²⁺.
(iii) The standard electrode potential for the M³⁺/M²⁺ half cell gives the relative stability between M³⁺ and M²⁺.
(iv) The high reduction potential of Mn³⁺/Mn²⁺ indicates Mn²⁺ is more stable than Mn³⁺.
(v) Mn³⁺ has a 3d⁴ configuration while that of Mn²⁺ is 3d⁵. The extra stability associated with a half filled d sub-shell makes the reduction of Mn³⁺ very feasible \(\left[\mathrm{E}^{\circ}=+1.51 \mathrm{~V}\right]\).

(i) The first transition metal Scandium exhibits only +3 oxidation state, but all other transition elements exhibit variable oxidation states by loosing electrons from (n-1)d orbital and ns orbital as the energy difference between them is very small.
(ii) At the beginning of the series, +3 oxidation state is stable but towards the end +2 oxidation state becomes stable.
(iii) The number of oxidation states increases with the number of electrons available, and it decreases as the number of paired electrons increases.
(iv) Hence, the first and last elements show less number of oxidation states and the middle elements with more number of oxidation states.
(v) For example, the first element Sc has only one oxidation state +3; the middle element Mn has six different oxidation states from +2 to +7. The last element Cu shows +1 and +2 oxidation states only.
(vi) The relative stability of different oxidation - states of 3d metals is correlated with the extra stability of half filled and fully filled electronic configurations. Example: Mn²⁺(3d⁵) is more stable than Mn⁴⁺(3d³).

(i) Ionization energy of transition element is intermediate between those of sand p block elements.
(ii) As we move from left to right in a transition metal series, the ionization enthalpy increases as expected.
(iii) This is due to increase in nuclear charge corresponding to the filling of d electrons.
(iv) The increase in first ionisation enthalpy with increase in atomic number along a particular series is not regular.
(v) The added electron enters (n-1)d orbital and the inner electrons act as a shield and decrease the effect of nuclear charge on valence ns electrons. Therefore, it leads to variation in the ionization energy values