(i) The high melting points of transition metals are due to strong metallic bonds between the atoms of these elements. This is because of involvements of a greater number of electrons in the interatomic metallic bonding from (n-l) d orbitals in addition to ns electrons.
(ii) Lanthanoids show limited the number of oxidation states, such as + 2, + 3 and + 4 (+ 3 is the principal oxidation state). This is because of the large energy gap between 5d and 4f subshells. On the other hand, actinoids also show principal oxidation state of + 3 but show a number of other oxidation states also. For example, uranium (Z = 92) exhibits oxidation states of + 3, + 4, + 5, + 6 and + 7 and neptunium (Z = 93) shows oxidation states of + 3, + 4, + 5, + 6 and + 7. This is because of the small energy difference between 5f, 6d and 7s orbitals
(iii) Most of the transition metal ions are colored both in the solid state and in aqueous solutions. The color of these ions is attributed to the presence of incomplete (n - 1) d-subshell. The electrons in these metal ions can be easily promoted from one energy level to another in the same d-subshell. The amount of energy required to excite the electrons to higher energy states within the same d-subshell corresponds to the energy of certain colors of visible light. Therefore, when white light falls on a transition metal compound, some of its energy corresponding to a certain color is absorbed causing promotion of d-electrons. This is known as d-d transitions. The remaining colors of white light are transmitted and the compound appears colored. For example, hydrated cupric compounds absorb radiations corresponding to red light and the transmitted color is greenish-blue (which is complementary color to red color). Thus, cupric compounds have greenish-blue color.

\(4 FeO. Cr_2O_3 + O_2\longrightarrow 2 Fe_2O_3 + 4 Cr_2O_3\)
\(\underline{4 Na_2CO_3 + 2 Cr_2O_3 + 3 O_2 \longrightarrow 4 Na_2CrO_4 + 4 CO_2 \times 2}\)_{_________________}
\(4 FeO. Cr_2O_3 + 8 Na_2CO_3 + 7 O_2\longrightarrow\underset{Sodium\ chromate} {\underset{{A}}{8 Na_2CrO_4}} + 2 Fe_2O_3 + 8 CO_2\)
\(2 Na_2CrO_4 + H_2SO_4 \longrightarrow\underset{(B)\\sodium\ choromate} {Na_2Cr_2O_7} + Na_2SO_4 + H_20\)

Transition elements are called d - block elements because the last electron enters (n - 1) d - orbital, i.e., d - orbital of the penultimate shell.

Using formula 
\(\mu=\sqrt{n(n+2)} B \cdot M ., n=3\)

Lanthanoids primarily show three oxidation states (+2, +3, +4). Among these oxidation states, +3 state is the most common. Lanthanoids display a limited number of oxidation states because the energy difference between 4f, 5d, and 6s orbitals is quite large. On the other hand, the energy difference between 5f, 6d, and 7s orbitals is very less. Hence, actinoids display a large number of oxidation states. For example, uranium and plutonium display +3, +4, +5, and +6 oxidation states while neptunium displays +3, +4, +5, and +7. The most common oxidation state in case of actinoids is also +3.

Elements (+ 2 state ) ₂₁Sc²⁺'₂₂Ti²⁺ ₂₃V²⁺ ₂₄ Cr²⁺ and ₂₅Mn²⁺have more stable +2 oxidation state and the outer electronic configuration are 3d¹ , 3d² , 3d³ , 3d⁴ and 3d⁵, respectively. In all the elements listed, the removal of two 4s electrons (in Cr²⁺, 1e^- from 4s and 1e^- from 3d), the 3d-orbitals get gradually occupied. Since, the number of empty d-orbitals decreases or the number of unpaired electrons in 3d-orbitals increases with increase in atomic number of cations, so the stability of the cations (M ²⁺) increases from Sc²⁺ to Mn.

(i) 6Fe²⁺+Cr₂O₇^2-+14H⁺\(\rightarrow \)6Fe³⁺+2Cr³⁺+7H₂O
(ii) 8MnO₄^-+3S₂O₃^2-+H₂O\(\rightarrow \)8MnO₂+6SO₄^2-+2OH^-

Ti³⁺, V³⁺, Mn²⁺, Fe³⁺ and Co²⁺ are coloured due to presence of unpaired electrons, they can undergo d-d transitions others  Cu⁺, Sc³⁺ are colourless due to absence of unpaired electrons.

La³⁺ is largest in lanthanoid series.

In actinoids, 5 f-orbitals are filled. These 5 f-orbitals have a poorer shielding effect than 4 f-orbitals (in lanthanoids). Thus, the effective nuclear charge experienced by electrons in valence shells in case of actinoids is much more than that experienced by electrons in valence shells in case of lanthanoids. Hence, the size contraction in actinoids is greater than that in lanthanoids.

(a) It is due to irregular variations in sum of first and second ionisation energies and sublimation energies. It is also due to stability of electronic configuration.
(b) (i) Cr₂O₇^2- + 3Sn²⁺ + 14H⁺➝ 2Cr³⁺ + 3Sn⁴⁺ + 7H₂O
(ii) MnO₄^- + 5Fe²⁺ + 8H⁺ ➝ SFe³⁺ + Mn²⁺ + 4H₂O