Chemistry Class 12 NCERT Solutions: Chapter 8 the D and F Block Elements Part 5 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Q: 19. Compare the stability of oxidation state for the elements of the first transition series.


Q_19_1_The Stability of + 2 Oxidation State
Sc+ 3
Ti+ 1+ 2+ 3+ 4
V+ 1+ 2+ 3+ 4+ 5
Cr+ 1+ 2+ 3+ 4+ 5+ 6
Mn+ 1+ 2+ 3+ 4+ 5+ 6+ 7
Q_19_2_The Stability of + 2 Oxidation State
Fe+ 1+ 2+ 3+ 4+ 5+ 6
Co+ 1+ 2+ 3+ 4+ 5
Ni+ 1+ 2+ 3+ 4
Cu+ 1+ 2+ 3
Zn+ 2

From the above table, it is evident that the maximum number of oxidation states is shown by , varying from . The number of oxidation states increases on moving from . On moving from , the number of oxidation states decreases due to a decrease in the number of available unpaired electrons. The relative stability of the oxidation state increases on moving from top to bottom. This is because on moving from top to bottom, it becomes more and more difficult to remove the third electron from the d-orbital.

Q: 20. Compare the chemistry of Actinoids with that of the lanthanoide with special reference to:

(i) Electronic configuration

(ii) Atomic and ionic sizes and

(iii) Oxidation state

(iv) Chemical reactivity


(i) Electronic configuration

The general electronic configuration for lanthanoids is and that for Actinoids is . Unlike orbitals, orbitals are not deeply buried and participate in bonding to a greater extent.

(ii) Atomic and ionic sizes and

Similar to lanthanoids, actinoids also exhibit actinoid contraction (overall decrease in atomic and ionic radii) . The contraction is greater due to the poor shielding effect of orbitals.

(iii) Oxidation state

The principal oxidation state of lanthanoids is . However, sometimes we also encounter oxidation states of and . This is because of extra stability of fully filled and half-filled orbitals. Actinoids exhibit a greater range of oxidation states. This is because the levels are of comparable energies. Again, is the principal oxidation state for actinoids. Actinoids such as lanthanoids have more compounds in state than in state.

(iv) Chemical Reactivity

In the lanthanide series, the earlier members of the series are more reactive. They have reactivity that is comparable to . With an increase in the atomic number, the lanthanides start behaving similar to . Actinoids, on the other hand, are highly reactive metals, especially when they are finely divided. When they are added to boiling water, they give a mixture of oxide and hydride. Actinoids combine with most of the non-metals at moderate temperatures. Alkalies have no action on these actinoids. In case of acids, they are slightly affected by nitric acid (because of the formation of a protective oxide layer) .

Q: 21. How would your account for the following:

(i) Of the species, is strongly reducing while manganese (III) is strongly oxidising.

(ii) Cobalt (II) is stable in aqueous solution but in the presence of complexing reagents it is easily oxidised.

(iii) The configuration is very unstable in ions.


(i) is strongly reducing in nature. It has a configuration. While acting as a reducing agent, it gets oxidized to (electronic configuration ) . This configuration can be written as configuration, which is a more stable configuration. In the case of , it acts as an oxidizing agent and gets reduced to . This has an exactly half-filled orbital and is highly stable.

(ii) (II) is stable in aqueous solutions. However, in the presence of strong field complexing reagents, it is oxidized to (III) . Although the 3rd ionization energy for is high, but the higher amount of crystal field stabilization energy (CFSE) released in the presence of strong field ligands overcomes this ionization energy.

(iii) The ions in configuration tend to lose one more electron to get into stable configuration. Also, the hydration or lattice energy is more than sufficient to remove the only electron present in the d-orbital of these ions. Therefore, they act as reducing agents

Q: 22. What is meant by ‘disproportionation’ ? Give two examples of disproportionation reaction in aqueous solution.


It is found that sometimes a relatively less stable oxidation state undergoes an oxidation − reduction reaction in which it is simultaneously oxidised and reduced. This is called disproportionation.

For example,


is oxidized to and reduced to


is oxidized to and reduced to

Q: 23. Which metal in the first series of transition metals exhibits oxidation state most frequently and why?


In the first transition series, exhibits oxidation state very frequently. It is because has an electronic configuration of . The completely filled makes it highly stable.

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