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

NCERT Solutions for Class 12 Chemistry Chapter 8 – Free PDF Download

*According to the CBSE Syllabus 2023-24, this chapter has been renumbered as Chapter 4.

NCERT Solutions for Class 12 Chemistry Chapter 8 The d and f Block Elements are powerful study materials that have answers to textbook exercises and important questions from previous years and sample papers. These NCERT Solutions for Class 12 Chemistry are prepared based on the latest CBSE Syllabus for 2023-24 and are given in a simple way for ease of understanding. Important tips and tricks have also been laid out along with these solutions.

The NCERT Solution for Class 12 Chemistry for the d and f block elements provided here can help the students clear all their doubts and understand the fundamentals better. Further, these solutions can be used for preparing chapter notes and during revisions. Download the free PDF of the NCERT Solutions for Class 12 Chemistry Chapter 8 from the link given below.

Download NCERT Solutions Class 12 Chemistry Chapter 8 PDF:-Download Here

Class 12 Chemistry NCERT Solutions Chapter 8 The d and f Block Elements – Important Questions

Q 8.1:

Write down the electronic configuration of

(i) Cr³⁺ (ii) Pm³⁺ (iii) Cu⁺ (iv) Ce⁴⁺ (v) Co²⁺ (vi) Lu²⁺ (vii) Mn²⁺  (viii) Th⁴⁺

Ans :

(i) Cr³⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d³

Or, [Ar]¹⁸ 3d³

(ii) Pm³⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 4f⁴

Or, [Xe]⁵⁴ 4f⁴

(iii) Cu⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰

Or, [Ar]¹⁸ 3d¹⁰

(iv) Ce⁴⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶

Or, [Xe]⁵⁴

(v) Co²⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁷

Or, [Ar]¹⁸ 3d⁷

(vi) Lu²⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 4f¹⁴ 5d¹

Or, [Xe]⁵⁴ 4f¹⁴ 5d¹

(vii) Mn²⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵

Or, [Ar]¹⁸ 3d⁵

(viii) Th⁴⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶

Or, [Rn]⁸⁶

Q 8.2:

Why are Mn²⁺ compounds more stable than Fe towards oxidation to their +3 state?

Ans : 

Electronic configuration:               Fe²⁺ is [Ar]¹⁸ 3d⁶.

Mn²⁺ is [Ar]¹⁸ 3d⁵.

We know that half and completely-filled orbitals are more stable. Hence, Mn with (+2) state has a stable d⁵ configuration, which is why Mn²⁺ shows resistance to oxidation to Mn³⁺. We know that Fe²⁺ has 3d⁶ configuration, and by losing one electron, its configuration changes to a more stable 3d⁵ configuration. Therefore, Fe²⁺ easily gets oxidized to the Fe⁺³ oxidation state.

Q 8.3:

Explain briefly how +2 state becomes more and more stable in the first half of the first-row transition elements with increasing atomic number.

Ans :

In the table below, the oxidation states of the first half of the first row of transition metals are displayed.

We can see that, except Sc, all other metals have a +2 oxidation state. As moving in the increasing order of atomic number from 21 to 25, that is, from Sc to Mn, the number of electrons in 3d orbitals increases from 1 to 5.

Sc (+2) = d¹

Ti (+2) = d²

V (+2) = d³

Cr (+2) = d⁴

Mn (+2) = d⁵

+2 oxidation state is attained by the loss of the two 4s electrons by these metals. Since the number of d electrons in (+2) state also increases from Ti(+2) to Mn(+ 2), the stability of +2 state increases (as the d-orbital is becoming more and more half-filled). Mn (+2) has d⁵ electrons (that is, a half-filled d shell, which is highly stable).

Q 8.4:

To what extent do the electronic configurations decide the stability of oxidation states in the first series of transition elements? Illustrate your answer with examples.

Ans :

The oxidation states with the maximum number (+2 to +7) are exhibited by the elements of Mn oxidation states that are in the first half of the transition series. Ti  With the increase in atomic number, the stability of the +2 oxidation state increases. Once more electrons are filled in the d-orbital, this happens to a great extent.  The +2 oxidation state is not shown by the Sc. Its electronic configuration is 4s² 3d¹. It loses all three electrons to form Sc³⁺, +3 oxidation state of Sc is very stable as by losing all three electrons, it attains stable noble gas configuration, [Ar], Ti (+ 4) and V(+5) are very stable for the same reason. For Mn, the +2 oxidation state is very stable as after losing two electrons, its d-orbital is exactly half-filled, [Ar] 3d⁵

Q 8.5:

What may be the stable oxidation state of the transition element with the following d electron configurations in the ground state of their atoms: 3d3, 3d5, 3d8 and 3d4?

Ans :

Q 8.6:

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

Ans :

(i) Chromate,

Oxidation state of Cr is + 6.

(ii) Permanganate,

Oxidation state of Mn is + 7.

(iii) Vanadate, 

Oxidation state of V is + 5.

Q 8.7:

What is lanthanoid contraction? What are the consequences of lanthanoid contraction?

Ans:

On moving along the lanthanoid series, the atomic number gradually increases by one. With the increase in atomic number, the number of protons and electrons present in the atom also increases by one. The effective nuclear charge increases as electrons are being added to the same shell. Owing to the proton addition being pronounced more above the interelectronic repulsions resulting from electron addition, a nuclear attraction increase happens. Also, with the increase in the atomic number, the number of electrons in the 4f orbital also increases. The 4f electrons have a 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 lanthanoids with an increase in the atomic number. This is termed lanthanoid contraction.

Consequences of lanthanoid contraction

(i) It is due to lanthanide contraction that there is variation in the basic strength of lanthanide hydroxides. (Basic strength decreases from La(OH)₃ to Lu(OH)₃.)

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

(iii) Separation of lanthanoids is possible due to lanthanide contraction.

Q 8.8:

What are the characteristics of the transition elements, and why are they called transition elements? Which of the d-block elements may not be regarded as transition elements?

Ans:

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

Elements such as Cd, Hg and Zn cannot be called transition elements because they have completely filled the d-sub shell.

Question 8.9:

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

Ans :

Transition metals have a partially filled d−orbital. Hence, the electronic configuration of transition elements is (n − 1)d^1-10 ns^0-2.

The non-transition elements either do not have a d−orbital or have a fully filled d−orbital.

Therefore, the electronic configuration of non-transition elements is ns^1-2 or ns² np^1-6.

Q 8.10:

What are the different oxidation states exhibited by lanthanoids?

Ans :

In the case of lanthanoids, the +3 oxidation state is the most common.

That is, Ln(III) compounds are predominant.

However, +2 and +4 oxidation states can also be found in the solution or in solid compounds.

Q 8.11:

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 catalysts.

Ans :

(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 a highly effective nuclear charge and a large number of valence electrons. Therefore, they form very strong metallic bonds. As a result, the enthalpy of atomisation of transition metals is high.

(iii) Most of the complexes of transition metals are coloured. This is because of the absorption of radiation from the visible light region to promote an electron from one of the d−orbitals to another. In the presence of ligands, the d-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) Transition metals provide a suitable surface for the reactions to occur.

(b) 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, Ea, for the reaction.

Q 8.12:

What are interstitial compounds? Why are such compounds well-known for transition metals?

Ans :

Transition metals are large in size, and they also contain lots of interstitial sites.

These interstitial sites can be used to trap atoms of other elements (that have small atomic sizes), such as H, C, N. The compounds resulting are called interstitial compounds.

Q 8.13:

How is the variability in oxidation states of transition metals different from that of non-transition metals? Illustrate with examples.

Ans :

In the case of transition elements, the oxidation state can vary from +1 to the highest oxidation state by removing all its valence electrons. Also, in transition elements, the oxidation states differ by 1 (Fe²⁺ and Fe³⁺; Cu⁺ and Cu²⁺). In non-transition elements, the oxidation states differ by 2, for example, +2 and +4 or +3 and +5, etc.

Q 8.14:

Describe the preparation of potassium dichromate from iron chromite ore. What is the effect of increasing pH on a solution of potassium dichromate?

Ans :

From iron chromite ore, Potassium dichromate is prepared as in the following steps.

Step (1): Preparation of sodium chromate

Step (2): Conversion of sodium chromate into sodium dichromate

Step (3): Conversion of sodium dichromate to potassium dichromate

Potassium chloride is less soluble than sodium chloride and is obtained in the form of orange-coloured crystals and can be removed by filtration.

The dichromate ion

However, by changing the pH, they can be interconverted.

Q 8.15:

Describe the oxidising action of potassium dichromate and write the ionic equations for its reaction with
(i) iodide (ii) iron(II) solution and (iii) H₂S

Ans:

(i)

(ii)

(iii)

Q 8.16:

Describe the preparation of potassium permanganate. How does the acidified permanganate solution react with

(i) iron(II) ions (ii) SO₂ and (iii) oxalic acid?

Write the ionic equations for the reactions.

Ans:

Potassium permanganate can be prepared from pyrolusite (MnO₂). The ore is fused with KOH in the presence of either atmospheric oxygen or an oxidising agent, such as KNO₃ or KClO₄, to give K₂MnO₄.

The green mass can be extracted with water and then oxidised either electrolytically or by passing chlorine/ozone into the solution. Electrolytic oxidation

At the anode, manganate ions are oxidised to permanganate ions.

Oxidation by chlorine

Oxidation by ozone

(i) Acidified KMnO₄ solution oxidises Fe (II) ions to Fe (III) ions, i.e., ferrous ions to ferric ions.

(ii). Acidified potassium permanganate oxidises SO₂ to sulphuric acid.

(iii) Acidified potassium permanganate oxidises oxalic acid to carbon dioxide

Q 8.17:

For M²⁺/M and M³⁺/M²⁺ systems, the EV values for some metals are as follows:
Cr²⁺/Cr -0.9V  Cr³/Cr²⁺ -0.4 V
Mn²⁺/Mn -1.2V  Mn³⁺/Mn²⁺ +1.5 V
Fe²⁺/Fe -0.4V Fe³⁺/Fe²⁺ +0.8 V

Use this data to comment upon:
(i) the stability of Fe³⁺ in acid solution as compared to that of Cr³⁺ or Mn³⁺ and
(ii) the ease with which iron can be oxidised as compared to a similar process for either chromium or manganese metal.

 Ans :

(i) The reduction potentials for the given pairs increase in the following order.

Mn²⁺ / Mn < Cr²⁺ / Cr < Fe²⁺ /Fe

So, the oxidation of Fe to Fe²⁺ is not as easy as the oxidation of Cr to Cr²⁺ and the oxidation of Mn to Mn²⁺. Thus, these metals can be arranged in the increasing order of their ability to get oxidised as Fe < Cr < Mn

(ii) The

Q 8.18:

Predict which of the following will be coloured in an aqueous solution. Ti³⁺, V³⁺, Cu⁺, Sc³⁺, Mn²⁺, Fe³⁺ and Co²⁺. Give reasons for each.

Ans :

Ions with electrons in the d-orbital will be the only ones that will be coloured, and the ions with empty d-orbital will be colourless.

All other ions, except Sc³⁺, will be coloured in an aqueous solution because of d−d transitions. Not Sc^3+, as it has an empty d-orbital.

Q 8.19:

Compare the stability of the +2 oxidation state for the elements of the first transition series.

 Ans:

 From the table above,

Mn shows the maximum number of oxidation states, changing between +2 to +7.

The number of oxidation states increases as we move on from Sc to Mn.

On moving from Mn to Zn, the number of oxidation states decreases due to a decrease in the number of available unpaired electrons.

The relative stability of the +2 oxidation state increases on moving from top to bottom. This is because, on moving to the bottom from the top, it becomes more and more difficult to remove the third electron from the d-orbital.

Question 8.20:

Compare the chemistry of actinoids with that of the lanthanoids with special reference to:
(i) electronic configuration (iii) oxidation state
(ii) atomic and ionic sizes and (iv) chemical reactivity

Ans :

(i) Electronic configuration

The general electronic configuration for lanthanoids is [Xe]⁵⁴ 4f^0-14 5d^0-1 6s^2, and that for actinoids is [Rn]⁸⁶ 5f^1-14 6d^0-1 7s². Unlike 4f orbitals, 5f orbitals are not deeply buried and participate in bonding to a greater extent.

(ii) Atomic and ionic sizes

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 5f orbitals.

(iii) Oxidation states

The principal oxidation state of lanthanoids is (+3). However, sometimes we also encounter oxidation states of + 2 and + 4. This is because of the extra stability of fully-filled and half-filled orbitals. Actinoids exhibit a greater range of oxidation states. This is because the 5f, 6d, and 7s levels are of comparable energies. Again, (+3) is the principal oxidation state for actinoids. Actinoids such as lanthanoids have more compounds in the +3 state than in the +4 state.

(iv) Chemical reactivity

In the lanthanide series, the earlier members of the series are more reactive. They have reactivity that is comparable to Ca. With an increase in the atomic number, the lanthanides start behaving similarly to Al. 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 non-metals at moderate temperatures. Alkalies have no action on these actinoids. In the case of acids, they are slightly affected by nitric acid (because of the formation of a protective oxide layer).

Question 8.21:

How would you account for the following?
(i) Of the d4 species, Cr²⁺ is strongly reducing while manganese(III) is strongly oxidising.
(ii) Cobalt(II) is stable in an aqueous solution, but in the presence of complexing reagents, it is easily oxidised.
(iii) The d1 configuration is very unstable in ions.

 Ans:

(i) Cr²⁺ is strongly reducing in nature. It has a d⁴ configuration. While acting as a reducing agent, it gets oxidised to Cr³⁺ (electronic configuration, d³). This d³ configuration can be written as configuration, which is a more stable configuration. In the case of Mn³⁺ (d⁴), it acts as an oxidising agent and gets reduced to Mn²⁺ (d⁵). This has an exactly half-filled d-orbital and is highly stable.

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

(iii) The ions in the d¹ configuration tend to lose one more electron to get into a stable d⁰ 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 8.22:

What is meant by ‘disproportionation’? Give two examples of disproportionation reactions in an aqueous solution.

Ans :

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,

(a)

Cr(V) is oxidised to Cr(VI) and reduced to Cr(III).

(b)

Mn (VI) is oxidized to Mn (VII) and reduced to Mn (IV).

Question 8.23:

Which metal in the first series of transition metals exhibits +1 oxidation state most frequently and why?

 Ans:

In the first series of transition metals, Cu exhibits +1 oxidation state very frequently. We know Cu has an electronic configuration of [Ar] 3d ¹⁰, that is, the completely filled d-orbital makes it highly stable.

Question 8.24:

Calculate the number of unpaired electrons in the following gaseous ions: Mn³⁺, Cr³⁺, V³⁺ and Ti³⁺. Which one of these is the most stable in an aqueous solution?

Ans:

Cr³⁺ is the most stable in aqueous solutions owing to a

Question 8.25:

Give examples and suggest reasons for the following features of transition metal chemistry.
(i) The lowest oxide of transition metal is basic, and the highest is amphoteric/acidic.
(ii) A transition metal exhibits the highest oxidation state in oxides and fluorides.
(iii) The highest oxidation state is exhibited in oxoanions of a metal.

Ans:

(i) In the case of a lower oxide of a transition metal, the metal atom has a low oxidation state. This means that some of the valence electrons of the metal atom are not involved in bonding. As a result, it can donate electrons and behave as a base. On the other hand, in the case of a higher oxide of a transition metal, the metal atom has a high oxidation state. This means that the valence electrons are involved in bonding, so they are unavailable. There is also a highly effective nuclear charge. As a result, it can accept electrons and behave as an acid.

For example,

(ii) Oxygen and fluorine act as strong oxidising agents because of their high electronegativities and small sizes. Hence, they bring out the highest oxidation states from the transition metals. In other words, a transition metal exhibits higher oxidation states in oxides and fluorides.

For example, in OsF₆ and V₂O₅, the oxidation states of Os and V are +6 and +5, respectively.

(iii) Oxygen is a strong oxidising agent due to its high electronegativity and small size. So, oxo-anions of metal have the highest oxidation state. For example, in, the oxidation state of Mn is +7.

Q 8.26:

Indicate the steps in the preparation of:
(i) K₂Cr₂O₇ from chromite ore. (ii) KMnO₄ from pyrolusite ore.

 Ans:

(i) Potassium dichromate ( K₂Cr₂O₇ ) is prepared from chromite ore (FeCr₂O₄) in the following steps.

Step (1): Preparation of sodium chromate

Step (2): Conversion of sodium chromate into sodium dichromate

Step(3): Conversion of sodium dichromate to potassium dichromate

Potassium chloride is less soluble than sodium chloride and is obtained in the form of orange-coloured crystals and can be removed by filtration.

The dichromate ion

However, by changing the pH, they can be interconverted.

(ii) Potassium permanganate ( KMnO₄ ) can be prepared from pyrolusite (MnO₂). The ore is fused with KOH in the presence of either atmospheric oxygen or an oxidising agent, such as KNO₃ or KClO₄, to give K₂MnO₄.

The green mass can be extracted with water and then oxidized either electrolytically or by passing chlorine/ozone into the solution. Electrolytic oxidation

At the anode, manganate ions are oxidised to permanganate ions.

Oxidation by chlorine

Oxidation by ozone

Question 8.27:

What are alloys? Name an important alloy which contains some of the lanthanoid metals. Mention its uses.

 Ans:

An alloy is a solid solution of two or more elements in a metallic matrix. It can be a partial solid solution or a complete solid solution. Alloys generally are created as they have varied properties compared to their constituent elements. An important alloy of lanthanoids is Mischmetal. It contains lanthanoids (94−95%), iron (5%), and traces of Al, Ca, Si, C, S.

Uses

(a) It is used in tracer bullets and shells.

(b) Mischmetal is used in cigarettes and gas lighters.

(c) It is used in flame-throwing tanks.

Question 8.28:

What are inner transition elements? Decide which of the following atomic numbers are the atomic numbers of the inner transition elements: 29, 59, 74, 95, 102, 104.

Ans :

Inner transition metals: Elements in which the last electron enters the f-orbital.

The elements in which the 4f and the 5f orbitals are progressively filled are called f-block elements.

From the given atomic numbers, the inner transition elements are the ones with atomic numbers 59, 95, and 102.

Q 8.29:

The chemistry of the actinoid elements is not as smooth as that of the lanthanoids. Justify this statement by giving some examples of the oxidation state of these elements.

Ans :

Lanthanoids project 3 oxidation states (+2, +3, +4). From these, the +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. While the energy difference between 5f, 6d, and 7s orbitals’ is very less. Hence, actinoids display a large number of oxidation states.

For example, neptunium displays +3, +4, +5, and +7, while uranium and plutonium display +3, +4, +5, and +6 oxidation states. The most common oxidation state in the case of actinoids is also +3.

Q 8.30 :

Which is the last element in the series of actinoids? Write the electronic configuration of this element. Comment on the possible oxidation state of this element.

 Ans :

In the actinoid series, the last element is lawrencium, Lr. The atomic number of the element is 103, and its electronic configuration is [Rn] 5f¹⁴ 6d¹ 7s². The most common oxidation state displayed by it is +3; because after losing 3 electrons, it attains a stable f¹⁴ configuration.

Q 8.31:

Use Hund’s rule to derive the electronic configuration of the Ce³⁺ ion, and calculate its magnetic moment on the basis of the ‘spin-only’ formula.

 Ans :

Magnetic moment can be calculated as

Where,

n = number of unpaired electrons

In Ce³⁺, n = 1

= 1. 732 BM

Q 8.32:

Name the members of the lanthanoid series which exhibit +4 oxidation states and those which exhibit +2 oxidation states. Try to correlate this type of behaviour with the electronic configurations of these elements.

Ans :

In the parenthesis are the atomic numbers of the elements given.

Tb, after forming Tb⁴⁺, attains a stable electronic configuration of [Xe] 4f⁷.

Yb, after forming Yb²⁺, attains a stable electronic configuration of [Xe] 4f¹⁴.

Eu, after forming Eu²⁺, attains a stable electronic configuration of [Xe] 4f⁷.

Ce, after forming Ce⁴⁺, attains a stable electronic configuration of [Xe].

Q 8.33 : 

Write the electronic configurations of the elements with the atomic numbers 61, 91, 101 and 109.

Ans:

Q 8.34:

Compare the general characteristics of the first series of 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

 Ans :

(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

Cu (29) = 3d¹⁰4s¹

Cr (24) = 3d⁵4s¹

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

Ag(47) = 4d¹⁰5s¹

Pd(46) = 4d¹⁰ 5s⁰

Rh(45) = 4d⁸5s¹

Ru (44) = 4d⁷5s¹

Tc (43) = 4d⁶5s¹

Mo (42) = 4d⁵5s¹

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

Au (79 ) = 5d¹⁰ 6s¹

Pt (78) = 5d⁹ 6s¹

W (74) = 5d⁴ 6s²

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, +2 and +3 oxidation states are quite stable for all elements present in the first transition series. All metals present in the first transition series form stable compounds in the +2 and +3 oxidation states. The stability of the +2 and +3 oxidation states decreases in the second and third transition series, wherein higher oxidation states are more important.

Some stable complexes are

The issue is that no such complexes are known for the second and third transition series, such as Mo, W, Rh, In. They form complexes in which their oxidation states are high.

For example: WCl₆, ReF₇, RuO₄, 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 4f 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 1^st transition series.

(iv) Atomic size generally decreases from left to right across a period. Now, among the three transition series, the 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.

Q 8.35:

Write down the number of 3d electrons in each of the following ions.

Ti²⁺, V²⁺, Cr³⁺, Mn²⁺, Fe²⁺, Fe³⁺, Co²⁺, Ni²⁺ and Cu²⁺.

Indicate how you would expect the five 3d orbitals to be occupied for these hydrated ions (octahedral).

Ans :

Q 8.36:

Comment on the statement that elements of the first transition series possess many properties different from those of heavier transition elements.

Ans:

In many ways, the properties of elements of heavier transition elements differ from those of the first transition series.

(a) The elements of the first transition series form low-spin or high-spin complexes depending upon the strength of the ligand field. However, the heavier transition elements form only low-spin complexes, irrespective of the strength of the ligand field.

(b) The enthalpies of atomisation of the elements in the first transition series are lower than those of the corresponding elements in the second and third transition series.

(c) The melting and boiling points of the first transition series are lower than those of the heavier transition elements. This is because of the occurrence of stronger metallic bonding (M−M bonding).

(d)  For heavier elements, the higher oxidation states are more common, whereas, for first transition series elements, th +2 and +3 oxidation states are more common.

(e) The atomic sizes of the elements of the first transition series are smaller than those of the heavier elements (elements of 2nd and 3rd 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.

Q 8.37:

What can be inferred from the magnetic moment values of the following complex species?
Example            Magnetic Moment (BM)
K₄[Mn(CN)₆)    2.2
[Fe(H₂O)₆]²⁺   5.3
K₂[MnCl₄]         5.9

Ans :

Magnetic moment  .

For value n = 1,

For value n = 2,

For value n = 3,

For value n = 4,

For value n = 5,

K₄[Mn(CN)₆]

For in transition metals, the magnetic moment is calculated from the spin-only formula.

Therefore, we can see from the above calculation that the given value is closest to n = 1. Also, in this complex, Mn is in the +2 oxidation state. This means that Mn has 5 electrons in the d-orbital.

Hence, we can say that CN⁻ is a strong field ligand that causes the pairing of electrons.

[Fe(H₂O)₆]²⁺

Thus, the calculation given above proves that the closest value to n= 4. In addition to this, the +2 oxidation state in this complex is none other than Fe. The d-orbital for Fe would have 6 electrons.

Thus, we can assume that H2O is a weak field ligand and does not induce electron pairing.

K₂[MnCl₄] 

From the equation, we can see that the given value is nearest to n = 5. Mn is in the +2 oxidation state in this complex. This implies that in the d-orbital, Mn has 5 electrons.

We may also assume that Cl – is a weak field ligand and does not induce electron pairing.

The elements lying in the middle of the periodic table between the s-block and p-block elements are known as d-block or transition elements. The f-block elements are called the inner-transition series. The d and f block elements introduce various important topics to the students, like general properties of the transition elements (d block), physical properties, variation in atomic and ionic sizes of transition metals, ionisation enthalpies, oxidation states, magnetic properties, the formation of coloured ions and complex compounds. To explore more about these concepts in detail, access the NCERT Solutions in this chapter.

Class 12 Chemistry NCERT Solutions for Chapter 8 d and f Block Elements

Chapter 8 d- and f-Block Elements of Class 12 Chemistry is framed as per the CBSE Syllabus for 2023-24. This chapter of NCERT Solutions for Class 12 deals with elements of the d and f blocks of the modern periodic table. In this chapter, students shall first deal with the electronic configuration, occurrence and general characteristics of transition elements with special emphasis on the trends in the properties of the first row (3d) transition metals, along with the preparation and properties of some important compounds. This will be followed by consideration of certain general aspects such as electronic configurations, oxidation states and chemical reactivity of the inner transition metals.

Subtopics of Chemistry Chapter 8 – The d and f Block Elements

1. Position in the Periodic Table
2. Electronic Configurations of the d-Block Elements
3. General Properties of the Transition Elements (d-Block)
4. Some Important Compounds of Transition Elements
5. The Lanthanoids
6. The Actinoids
7. Some Applications of d- and f-Block Elements

Why Opt for BYJU’S?

While solving the Class 12 Chemistry NCERT textbook questions of Chapter 8 with the help of NCERT Solutions for Class 12 Chemistry Chapter 8, students should also solve lots of sample papers and previous years’ questions. Solving previous years’ questions and sample papers will help students to get acquainted with the latest exam pattern as well as the marking scheme.

Along with NCERT Solutions, students should also follow some good study materials; NCERT textbooks are one of the best study materials for students who are studying in Class 12. Students are also advised to make study notes so that they can go through the notes for quick revision.