(i) (a) In p-block, elements show variable oxidation state. It increases as we move from left to right in the periodic table. The maximum oxidation state shown by p-block element is equal to the total number of valence electrons. Whereas in d-block, elements show different oxidation states because of incomplete d-subshell. The variable oxidation state is due to the participation of ns and (n -1) d electrons in bonding.
(b) In aqueous solution, Cu⁺ ion undergoes disproportionation reaction.
2Cu⁺ (aq) ⇾ Cu²⁺ + Cu(s)
The stability of Cu²⁺ ion in aqueopus solution is due to negative enthalpy of hydration. which more compensate for the IE₂ or Cu.
(c) When orange solution containing \(Cr_2O_7^{2-}\) ion is treated with an alkali, a yellow solution of  (Chromate ion) is obtained.
\(\underset{Dichromate\\(Orange)}{Cr_2O_7^{2-}}\xrightarrow{OH^-}\underset{chromate\ ion\\(Yellow)}{Cr_2O_4^{2-}}\)
(ii) Chemistry of actinoids is complicated as compared to the lanthanoids. The two reasons are:
(a) 5-orbital present in actinoids is more exposed to the outer environment while 4-f orbital present in lanthanoide are deeply buried.
(b) Lanthanoids show limited number of oxidation states as +2,+3 and + 4 (out of which + 3is most common). This is-due to the large energy gap between 4f and 5d subshells. The dominant oxidation state of actinoids is also + 3 but they show a number of other oxidation states also like uranium (Z = 92) and plutonium (Z = 94) show +3, + 4, + 5and +6, neptunium shows +3, + 4, + 5 and +7. This is due to small energy difference between 5f, 6d, and 7s subshells.

(i) (a) Due to the comparatively smaller size of the metal ions, their high ionic charges and the availability of vacant d-orbitals for bond formation, transition metals form a  large number of complex compounds.
(b) As oxidation number (or oxidation state) of an element increases ionic character decreases. In general, the oxides in lower oxidation states of metals are basic and in their higher oxidation state, the oxides are amphoteric.
In lower oxidation state of the metal, some of the valence electrons of the metal atom are not involved in bonding. Hence, it can donate electrons and behave as a base. In higher oxidation state, valence electrons are involved in bonding and hence, electrons are not available for donation. Instead, their effective nuclear charge is high and hence they behave as acids.
(c) Mn³⁺(3d⁴) is less stable than Mn²⁺(3d⁵) because Mn²⁺ has stable half-filled configuration. Cr³⁺ has stable 3d³(t³_2g) configuration, therefore, Cr³⁺ cannot be reduced to Cr²⁺. That's why, EO value for the Mn³⁺ / Mn²⁺ couple is much more positive than Cr³⁺ /Cr²⁺. In other words, Mn³⁺ is a strong oxidising agent.
(ii) Similarity Both lanthanoids and actinoids exhibit +3 oxidation state predominantly.Difference Lanthanoids have less tendency towards complex formation while actinoids have greater tendency towards complex formation.

(i) Mn = 3d⁵4s²
So, Mn³⁺ = 3d⁴

Number of unpaired electrons = 4
Cr=3d⁵4s¹ Cr³⁺ = 3d³

Number of unpaired electrons: 3
Y = 3d³4s² y³⁺ = 3d²

Number of unpaired electrons = 2
Fe= 3d⁶4s² , Fe²⁺ = 3d⁶

Number of unpaired electrons = 4
Mn³⁺ will be most stable because being smallest in size, it has maximum  hydration energy and hence, more stability.

The information suggests that the gas (A) is H₂S while the gas (B)is S0₂ Both turn acidified K₂Cr₂O₇ paper green
K₂Cr₂O₇ + 4H₂SO₄ + 3H₂S \(\longrightarrow \) K₂SO₄ + Cr₂(SO₄)₃+7H₂O + 3S
                                 (gas A)                      green
K₂Cr₂O₇ + H₂SO₄ + 3SO₂ \(\longrightarrow \) K₂SO₄ + Cr₂(SO₄)₃
                                 (gas B)                      green
H₂S + (CH₃COO)₂Pb \(\longrightarrow \) PbS + 2CH₃COOH
(gas A)                             Black ppt
2H₂S + SO₂ \(\longrightarrow \)  2H₂O + 3S
(gas A) (gas B)    (Yellowish turbidity)

(i) The transition elements exhibit variable oxidation states. The variable oxidation states of transition metals are due to the participation of ns and (n - 1) d-electrons. This is because of the very small difference between the energies of (n - 1) d and ns orbitals. For the first five elements, the minimum oxidation state is equal to the number of electrons ---in the 4s orbitals and the other oxidation states are equal to the sum of 4s and some of the 3d-electrons. The highest oxidation state is equal to the sum of 4s and 3d electrons. For the remaining elements, the minimum oxidation state is equal to electrons in 4s-orbitals and the maximum oxidation state is not equal to the sum of 4s and 3d electrons. In general, the oxidation state increases up to the middle and then decreases.

(a) (i) This is due to increase stability of lower species to which they are reduced.
(ii) The electronic configuration of manganese is 3^d548z. After the loss of the outer 48 electrons; its electronic configuration becomes stable because of half filled configuration. Therefore, it becomes difficult to remove the third electrons and hence its third ionization enthalpy is exceptionally high.
(iii) Cr²⁺ is reducing because its configuration changes from 3^d4 to 3d³. The 3d³ configuration of Cr²⁺in its compounds (expressed as t2 3) is stable because it has half filled t²g subshell. In the other hand, Fe²⁺on changing to Fe³⁺becomes 3^d5 which is not as stable as 3d3 in its compounds. Therefore, Cr²⁺ is stronger reducing than Fe²⁺.

(i) (a) CuF2 (b) (i) Cr in crO²₄ - is in the highest oxidation state, i.e., +6 while Mn in MnO is in oxidation state +6 and its most stable oxidation state is +7
(ii) due to lanthanoid contraction
(iii) [Mn²⁺ has 3d 5 configuration]

(i) Electronic configurations: In 1^st transition series, 3d orbitals are progressively filled, whereas, in 2nd transition series, 4d orbitals are progressively filled and in 3^rd transition series, 5d-orbitals are progressively filled.
(ii) Oxidation states: Elements show variable oxidation states in both the. series. The highest oxidation state is equal to a total number of electrons in '5' as well as 'd' orbitals. The number of oxidation states shown is less in 5d transition series than 4d series. In 3d series +2, +3 oxidation states are common and they form stable complexes in these oxidation states. In other series, SO₄ and PtF₆ are formed which are quite stable in higher oxidation state. 
(iii) ionization enthalpies: The ionization enthalpy of 5d series is higher than 3d and 4d series due to lanthanide contraction, the effective nuclear charge is more.
(iv) Atomic sizes: The atomic sizes of 4d and 5d series do not differ appreciably due to lanthanoid contraction. The atomic radii of second and third series are larger than 3d series.

(a) (i) covalent radii: The atomic radii decrease from 5c to Mn because of a number of unpaired electrons increases, therefore, effective nuclear charge increases. The atomic size of Fe, Co, Ni is almost same because the pairing of electrons takes place in d-orbitals causing repulsion and effective nuclear charge does not increase appreciably. Cu and In have bigger size because repulsion between paired electrons increases. Ionic radii of bivalent cations decrease from 5c to Cu due to increase in a number of protons.
(ii) Oxidation states: Transition metals show variable oxidation states due to the tendency of 'd' as well as '5' electrons to take part in bond formation. The highest oxidation state is equal to the total number of electrons in '5' as well as d-orbitals. The maximum oxidation state shown by the elements of first transition series increases from 5c to Mn and then decreases to In. 5c shows maximum + 3 and Mn shows +7, V(+ 5), Cr(+ 6), Fe(+3), Ni(+2), Co(+3), Cu(+2) and In( +2). oxidation state.
(iii) Ionization enthalpies: There is slight and irregular variation in ionization energies of transition metals due to the irregular variation of atomic size. The I.E. of 5d transition series is higher than 3d and 4d transition series because of Lanthanoid contraction, effective nuclear charge increases.
(b) Misch metal is an alloy which contains some of the lanthanoid metals. It contains 45% lanthanoid metals and iron ~ 5% and traces of 5, C, Ca and AI. Misch metal is used in the Mg-based alloy to produce bullets, shell, and lighter flint. Addition of 3% misch metal to magnesium increases its strength and used in making jet engine parts

Mn³⁺ = 3d⁴ = 4 unpaired electron, Cr³⁺ = 3d³ = 3 unpaired electrons, V³⁺ = 3d² = 2 unpaired electrons, Ti³⁺= 3d ¹ = 1 unpaired electron. Cr³⁺ is most stable out of these in aqueous solution because it has half filled t_2g level (i.e., t³_2g) .