Class 12 Chemistry Important Chapter 8 The d- and f-Block Elements

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Class 12 Chemistry Important Chapter 8 The d- and f-Block Elements

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The d- and f-Block Elements

Chapter: 8

PART – I

IMPORTANT QUESTION AND ANSWER

1. What are transition elements?

Ans: Transition elements are those which have partially filled d-orbitals either in their atomic or ionic states. According to IUPAC, transition metals are defined as metals which have incomplete d subshells in at least one of their oxidation states.

2. Compare the stability of Fe²⁺ and Fe³⁺ ions in aqueous solution. Which is more stable and why?

Ans: Fe³⁺ is more stable in aqueous solution than Fe²⁺ because of its smaller ionic radius and higher hydration enthalpy. Also, Fe³⁺ has a half-filled t₂g orbital (d⁵) configuration in octahedral crystal field, which gives extra stability due to exchange energy.

3. Which series of elements belong to the d-block? 

Ans: The d-block elements include:

3d series (Sc to Zn).

4d series (Y to Cd).

5d series (La and Hf to Hg).

6d series (Ac and Rf to Cn).

4. Which metal in the first series transition metals exhibits + 1 oxidation state m frequently and why?Ans. Cu (at no. 29) of first transition series C exhibits +1 oxidation state.

Ans: It is due to completely filled 3d orbitals in state.

5. Which elements are known as inner transition metals?

Ans: The inner transition metals consist of the:

4f block (Lanthanoids: Ce to Lu).

5f block (Actinoids: Th to Lr).

6. Why is Zn²⁺ colourless, whereas Cu²⁺ is blue in aqueous solution?

Ans: Zn²⁺ has a d¹⁰ configuration (completely filled d-orbitals), so no d–d transitions are possible, making it colourless. Cu²⁺, with a d⁹ configuration, allows d–d transitions by absorbing visible light, resulting in blue colour due to the complementary colour being observed.

7. Why does Mn exhibit the highest number of oxidation states among the 3d-series elements?

Ans: Manganese has a configuration of [Ar] 3d⁵ 4s². Since it has five unpaired d-electrons, it can lose different numbers of electrons (from both d and s orbitals), allowing it to show oxidation states from +2 to +7. The maximum number of unpaired electrons gives maximum oxidation variability.

8. What are some key physical properties of transition metals?

Ans: (i) High melting and boiling points.

(ii) High density.

(iii) Good electrical and thermal conductivity.

(iv) Exhibit metallic luster.

(v) Hard and form alloys.

9. Why do transition elements form coloured compounds?

Ans: Because of the presence of unpaired d-electrons which undergo d–d electronic transitions, absorbing specific wavelengths of visible light and reflecting the complementary colour.

10. What is meant by lanthanoid contraction?

Ans: It refers to the steady decrease in atomic and ionic sizes of the lanthanoids (from La to Lu) due to poor shielding effect of the 4f electrons.

11. Give one example of a transition metal with maximum oxidation states.

Ans: Manganese (Mn) shows oxidation states from +2 to +7, the highest in the 3d series.

12. Name a strong oxidising agent among the 3d metals and why.

Ans: MnO₄⁻ (permanganate ion) is a strong oxidising agent due to the high oxidation state of Mn (+7) and its tendency to get reduced to a more stable Mn²⁺ state.

13. Why is Scandium (Sc) not variable in its oxidation state?

Ans: Scandium commonly exhibits only the +3 oxidation state due to its electronic configuration (3d¹4s²), and further removal of electrons is not energetically favorable.

14. Assign a reason for each of the following observations:

(i) The transition metals (with the exception of Zn, Cd and Hg) are hard and have high melting and boiling points.

Ans: Atoms of the transition elements are closely packed and held together by strong metallic bonds which have appreciable covalent character. This leads to high melting and boiling points of the transition elements.

The electronic configuration of these elements reveals that they have one or more unpaired electrons present in either ns or (n^{– 1})d orbitals, which are available for bond formation. In general, greater the number of such electrons available, more will be the chances of their mutual combination and more will be the strength of the metallic bond. That is why these metals are very hard.

(ii) The ionisation enthalpies (first and second) in the first series of the transition elements are found to very irregularly. 

Ams: The ionisation enthalpy increases due to the increase in the nuclear charge with atomic number at the beginning of the series. Gradually, the shielding effect of the added electrons also increases. This shielding effect tends to decrease the attraction due to the nuclear charge. These two opposing factors lead to a rather gradual increase in the ionisation enthalpies. Thus the ionisation enthalpies in the first series of the transition elements are found to vary irregularly.