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

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Q: 33. Compare the chemistry of the actinoids with that of lanthanoids with reference to:

(i) Electronic Configuration

(ii) Oxidation states and

(iii) Chemical Reactivity

Answer:

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.

Oxidation states:

The principal oxidation state of lanthanoids is . However, sometimes we also encounter oxidation states of and . This is because 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.

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: 34. Write the electronic configurations of the elements with the atomic numbers .

Answer:

Q_34_The Electronic Configurations
Q_34_The Electronic Configurations

Atomic number

Electronic configuration

61

91

101

109

Q: 35. Compare the general characteristics of the first series of the transition metals with those of the second and third series metals in the respective vertical columns. Give special emphasis on the following points:

(i) electronic configurations,

(ii) oxidation states,

(iii) ionisation enthalpies, and

(iv) atomic sizes

Answer:

(i) In the 1st, 2nd and 3rd transition series, the 3d, 4d and 5d orbitals are respectively filled.

We know that elements in the same vertical column generally have similar electronic configurations.

In the first transition series, two elements show unusual electronic configurations:

Similarly, there are exceptions in the second transition series. These are:

There are some exceptions in the third transition series as well. These are:

As a result of these exceptions, it happens many times that the electronic configurations of the elements present in the same group are dissimilar.

(ii) In each of the three transition series the number of oxidation states shown by the elements is the maximum in the middle and the minimum at the extreme ends.

However, oxidation states. The stability of the oxidation states decreases in the second and the third transition series, wherein higher oxidation states are more important.

For example , are stable complexes, but no such complexes are known for the second and third transition series such as . They form complexes in which their oxidation states are high. For example: , etc.

(iii) In each of the three transition series, the first ionisation enthalpy increases from left to right. However, there are some exceptions. The first ionisation enthalpies of the third transition series are higher than those of the first and second transition series. This occurs due to the poor shielding effect of electrons in the third transition series. Certain elements in the second transition series have higher first ionisation enthalpies than elements corresponding to the same vertical column in the first transition series.

There are also elements in the 2nd transition series whose first ionisation enthalpies are lower than those of the elements corresponding to the same vertical column in the 1st transition series.

(iv) Atomic size generally decreases from left to right across a period. Now, among the three transition series, atomic sizes of the elements in the second transition series are greater than those of the elements corresponding to the same vertical column in the first transition series. However, the atomic sizes of the elements in the third transition series are virtually the same as those of the corresponding members in the second transition series. This is due to lanthanoid contraction.