Chemistry Class 12 NCERT Solutions: Chapter 8 The d and f Block Elements Part 2

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Q: 6. Name the Oxometal anions of the first series of the transition metals in which the metal exhibits the oxidation state equal to its group number.

Answer:

(i) Vanadate,

Oxidation state of

(ii) Chromate,

Oxidation state of .

(iii) Permanganate,

Oxidation state of .

Q: 7. What is lanthanoid contraction? What are the consequences of lanthanoid contraction?

Answer:

As we move along the lanthanoid series, the atomic number increases gradually by one. This means that the number of electrons and protons present in an atom also increases by one. As electrons are being added to the same shell, the effective nuclear charge increases. This happens because the increase in nuclear attraction due to the addition of proton is more pronounced than the increase in the interelectronic repulsions due to the addition of electron. Also, with the increase in atomic number, the number of electrons in the orbital also increases. The electrons have poor shielding effect. Therefore, the effective nuclear charge experienced by the outer electrons increases. Consequently, the attraction of the nucleus for the outermost electrons increases. This results in a steady decrease in the size of lanthanide with the increase in the atomic number. This is termed as lanthanoid contraction.

Consequences of lanthanoid contraction

(i) There is similarity in the properties of second and third transition series.

(ii) Separation of lanthanide is possible due to lanthanide contraction.

(iii) It is due to lanthanide contraction that there is variation in the basic strength of lanthanide hydroxides. (Basic strength decreases from )

Q: 8. What are the characteristics of the transition elements and why are they called transition elements? Which of the block elements may not be regarded as the transition elements?

Answer:

Transition elements are those elements in which the atoms or ions (in stable oxidation state) contain partially filled orbital. These elements lie in the block and show a transition of properties between and . Therefore, these are called transition elements.

Elements such as cannot be classified as transition elements because these have completely filled .

Q: 9. In what way is the electronic configuration of the transition elements different from that of the non-transition elements?

Answer:

Transition metals have a partially filled -orbital. Therefore, the electronic configuration of transition elements is .

The non-transition elements either do not have a orbital or have a fully filled−orbital. Therefore, the electronic configuration of non-transition elements is .

Q: 10. What are the different oxidation states exhibited by the lanthanide?

Answer:

In the lanthanide series, oxidation state is most common i.e., compounds are predominant. However, and oxidation states can also be found in the solution or in solid compounds.

Q: 11. Explain giving reasons:

(i) Transition metals and many of their compounds show paramagnetic behaviour.

(ii) The enthalpies of atomisation of the transition metals are high.

(iii) The transition metals generally form coloured compounds.

(iv) Transition metals and their many compounds act as good catalyst.

Answer:

(i) Transition metals show paramagnetic behaviour. Paramagnetism arises due to the presence of unpaired electrons with each electron having a magnetic moment associated with its spin angular momentum and orbital angular momentum. However, in the first transition series, the orbital angular momentum is quenched. Therefore, the resulting paramagnetism is only because of the unpaired electron.

(ii) Transition elements have high effective nuclear charge and a large number of valence electrons. Therefore, they form very strong metallic bonds. As a result, the enthalpy of atomization of transition metals is high.

(iii) Most of the complexes of transition metals are coloured. This is because of the absorption of radiation from visible light region to promote an electron from one of the orbitals to another. In the presence of ligands, the orbitals split up into two sets of orbitals having different energies. Therefore, the transition of electrons can take place from one set to another. The energy required for these transitions is quite small and falls in the visible region of radiation. The ions of transition metals absorb the radiation of a particular wavelength and the rest is reflected, imparting colour to the solution.

(iv) The catalytic activity of the transition elements can be explained by two basic facts.

(a) Owing to their ability to show variable oxidation states and form complexes, transition metals form unstable intermediate compounds. Thus, they provide a new path with lower activation energy, , for the reaction.

(b) Transition metals also provide a suitable surface for the reactions to occur.

Q: 12. What are interstitial compounds? Why are such compounds well known for transition metals?

Answer:

Transition metals are large in size and contain lots of interstitial sites. Transition elements can trap atoms of other elements (that have small atomic size), such as , in the interstitial sites of their crystal lattices. The resulting compounds are called interstitial compounds.