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

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According to definition, transition elements are those which have partially filled d-subshells in their elementary state or in their commonly occurring oxidation states. Scandium (Z = 21) has partially filled d-subshell (3d¹) and is therefore, a transition element. On the other hand, zinc (Z = 30) has the configuration 3d¹⁰4s². It does not have partially filled d-subshell in its elementary form or in commonly occurring oxidation state (Zn²⁺ : 3d¹⁰). Therefore, it is not regarded as transition element.

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The high enthalpies of atomization are due to large number of unpaired electrons in their atoms. Therefore, they have stronger interatomic interactions and hence, stronger bonding between atoms. Thus, they have high enthalpies of atomization.

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Scandium (Z = 21) does not exhibit variable oxidation states.

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This is due to the increasing stability of the lower species to which they are reduced.

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Divalent ion in aqueous solution having atomic number 25 will have d5 configuration.
No. of unpaired electron = 5

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When a particular oxidation state becomes less stable relative to other oxidation states, one lower and one higher, it is said to undergo disproportionation. For example, manganese (VI) becomes unstable relative to manganese (VII) and manganese (IV) in acidic solution

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Cerium (Z = 58)

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According to definition, transition elements are those which have partially filled d-subshell in their elementary state or in their one of the oxidation states. Silver (Z = 47) can exhibit +2 oxidation state in which it has incompletely filled d-subshell (4d⁹ configuration). Hence, silver is regarded as transition element.

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The high enthalpies of atomization of transition elements is due to the participation of electrons (n–1) d-orbitals in addition to ns electrons in the interatomic metallic bonding. In case of zinc, no electrons from 3d-orbitals are involved in the formation of metallic bonds. Therefore, its enthalpy of atomisation is the lowest. On the other hand, in all other metals of 3d series electrons from d-orbitals are always involved in the formation of metallic bonds.

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Mn (Z = 25) exhibits the largest number of oxidation states because it has the maximum number of unpaired electrons. Hence, it shows oxidation states from +2 to +7 i.e. +2, +3, +4, +5, +6 and +7.

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The copper has high enthalpy of atomisation and enthalpy of ionization. Therefore, the high energy required to transform Cu(s) to Cu2+ (aq) is not balanced by its low enthalpy of hydration.

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The irregular trend in variation of ionisation enthalpies in the first series of transition elements is because of varying degree of stability of different configurations.

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Because oxygen and fluorine have small size and high electronegativity, therefore, they can easily oxidise the metal to its highest oxidation state.

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Cr²⁺ is a stronger reducing agent than Fe²⁺. This is because the configuration of Cr²⁺ changes from d⁴ to d³ and d3 configuration is stable (t_2g³) being half filled t_2g level.

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The electronic configuration of M²⁺ (Z = 27)
is M²⁺ (Z = 27) : 3d⁷

It has 3 unpaired electrons. The spin only magnetic moment is

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Cu⁺(aq) is not stable in aqueous solution because of its less negative enthalpy of hydration than of Cu²⁺ (aq).

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This is because of relatively poor shielding by 5f electrons in actinoids in comparison with shielding of 4f electrons in lanthanoids.

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Electronic configuration of Mn²⁺ is [Ar] 3d⁵ which is half filled and hence it is stable. Therefore, third ionisation enthalpy is very high i.e., third electron cannot be easily removed. In case of Fe²⁺, the electronic configuration is 3d6. Therefore, Fe²⁺ can easily lose one electron to acquire 3d⁵ stable electronic configuration.

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As we move in the first row transition elements, the sum of IE1 and IE2 increases. As a result reduction potential (E$°$) becomes less and less negative and hence the stability decreases upto first half of the first row elements.

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In a transition series, the oxidation states which result in half filled and completely filled of d-subshell are more stable. For example, in the first transition series, electronic configuration of Mn (Z = 25) is [Ar] 3d⁵ 4s². It shows oxidation states from +2 to +7 but Mn (II) is most stable because it has stable electronic configuration. Similarly, we can say Zn (Z = 30) having electronic configuration [Ar] 3d¹⁰4s² exhibits stable +2 oxidation state because of stable completely filled 3d¹⁰ configuration.

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The stable oxidation states are predicted on the basis that up to Mn, the maximum oxidation states of stability correspond to sum of s and d-electrons. After Mn, there is decrease in the stability of higher oxidation states.

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MnO₄^– : Oxidation state of Mn = + 7 (Group No = 7)
CrO₄^2–: Oxidation state of Cr = + 6 (Group No = 6)

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Lanthanoid contraction. In the lanthanoids there is regular decrease in the size of atoms and ions with increase in atomic number. This decrease of ionic and atomic radii in the lanthanoid elements is called lanthanoid contraction. For example, the ionic radii decrease from Ce³⁺ (111 pm) to Lu³⁺ (93 pm). Cause of lanthanoid contraction. As we move through the lanthanoid series, 4f-electrons are being added, one at each step. The mutual shielding effect of electrons is very little, even smaller than that of d-electrons. This is due to the shape of the f-orbitals. The nuclear charge, however, increases by one at each step. Hence, the inward pull experienced by the 4f-electrons increases. This causes a reduction in the size of the entire 4fn shell. The sum of the successive reductions gives the total lanthanoid contraction.

Consequences of lanthanoid contraction. The important consequences of lanthanoid contraction are :

(i) It has very important effect on the relative properties of the elements which follow the lanthanoids with those which proceed the lanthanoids. We know that there is a regular increase in size as we go from Sc to Y to La in group 3. Similarly, we expect normal, increase in size in other groups from Ti $\to$ Zr $\to$ Hf
V $\to$ Nb $\to$ Ta
However after lanthanoids, the increase in radii from second to third transition series vanishes.
Consequently, the pairs of elements Zr—Hf,
Nb—Ta, Mo—W, etc. have almost similar sizes.
Therefore, the properties of these elements are similar.

(ii) There is small but steady increase in the standard electrode potential values (E$°$) for the process :
M³⁺ + 3e– $\to$ M(g)

(iii) The basic strength of oxides and hydroxides decrease with increase in atomic number. Thus, La(OH)₃ is most basic and Lu(OH)₃ is least basic.

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The general characteristics of d-block elements are :

1. Nearly all the transition elements have typical metallic properties such have high tensile strength, ductility, malleability, high thermal and electrical conductivity and metallic lustre.
2. Except mercury which is liquid at room temperature, other transition elements have typical metallic structures.
3. They have high melting and boiling points and have higher heats of vaporisation than non-transition elements.
4. They are electropositive in nature.
5. Most of them form coloured compounds.
6. They have good tendency to form complexes.
7. They exhibit several oxidation states.
8. Their compounds are generally paramagnetic in nature.
9. They form alloys with other metals.
10. Most of the transition metals and other compounds are used as catalysts.

They are called transition elements because they represent transition (change) in properties from most electropositive s-block elements to most electronegative p-block elements.

The elements Zn, Cd and Hg are not regarded as transition elements because they donot have partly filled d-subshells in their elementary state or in their commonly occurring oxidation states.

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The transition elements involve the filling of d-orbitals while the representative elements involve the filling of s or p-orbitals. The general electronic configuration of transition elements is (n–1) d^1–10 ns^1–2.

On the other hand, the general electronic configuration of representative elements in ns^1–2 or ns² np^1–6. Thus, in representative elements, only the last shell is incomplete while in transition elements the last but one shell is also incomplete.

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The principal oxidation state of lanthanoids is + 3. In addition they exhibit oxidation states of + 2 and + 4.

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1. Most of the compounds of transition elements contain unpaired electrons in their (n–1) d-subshells. Therefore, they are paramagnetic in nature and are attracted by the magnetic field. The magnetic character is expressed in terms of magnetic moment. The larger the number of unpaired electrons in a substance, the greater is the paramagnetic character and larger is the magnetic moment. The magnetic moment is expressed in Bohr magneton abbreviated as B.M.
   For example, Ti²⁺ has 2 unpaired electrons and has less magnetic moment than V²⁺ which has 3 unpaired electrons. Mn²⁺ has 5 unpaired electrons and has maximum magnetic moment among the divalent transition metal ions because d-subshell can have maximum of 5 unpaired electrons.
2. The high enthalpies of atomization are due to large number of unpaired electrons in their atoms. Therefore, they have stronger interatomic interactions and hence, stronger bonding between atoms. Thus, they have high enthalpies of atomization.
3. Most of the transition metal ions are coloured both in the solid state and in aqueous solutions. The colour of these ions is attributed to the presence of incomplete (n – 1) d-subshell. The electrons in these metal ions can be easily promoted from one energy level to another in the same d-subshell. The amount of energy required to excite the electrons to higher energy states within the same d-subshell corresponds to energy of certain colours of visible light. Therefore, when white light falls on a transition metal compound, some of its energy corresponding to a certain colour, is absorbed causing promotion of d-electrons. This is known as d-d transitions. The remaining colours of white light are transmitted and the compound appears coloured.
   For example, hydrated cupric compounds absorb radiations corresponding to red light and the transmitted colour is greenish blue (which is complementry colour to red colour). Thus, cupric compounds have greenish blue colour.
   It may be noted that transition metal ions containing completely filled d-orbitals (i.e., d10 configuration) such as Zn²⁺, Cd²⁺, Hg²⁺, Cu⁺, etc. are generally white. For example, ZnCl₂ is white.
4. Some transition metals and their compounds act as good catalysts for various reactions. This is due to their ability to show multiple oxidation states. The common examples are Fe, Co, Ni, V, Cr, Mn, Pt, etc.
   The transition metals form reaction intermediates with the substrate by using empty d-orbitals. These intermediates give reaction paths of lower activation energy and therefore, increase the rate of reaction. For example, during the conversion of SO₂ to SO₃, V₂O₅ is used as a catalyst. Solid V₂O₅ absorbs a molecule of SO₂ on the surface forming V₂O₄ and the oxygen is given to SO2 to form SO₃. The divanadium tetraoxide is then converted to V₂O₅ by reaction with oxygen:

   

   2V2O4 + O2 $\to$ 2V2O5

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The crystal lattices of transition metals have vacant interstitial sites into which small sized non-metal atoms (H, C, N, B) can easily fit, resulting in the formation of compounds known as interstitial compounds. These are usually non-stoichiometric and are neither typically ionic nor covalent. For example, TiC, Mn₄H, Fe₃H, TiH_1.7, etc. They have chemical properties similar to that of the parent metal but different physical properties such as hardness, electrical conductivity, etc.

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The variability of oxidation states in transition elements arise because of the participation of (n–1) d-orbitals and ns orbitals. These different oxidation states of transition elements differ by unity. For example, vanadium shows oxidation states of + 2, + 3, + 4 and + 5; manganese shows oxidation states of + 2, + 3, + 4, + 5, + 6 and + 7. On the other hand, some non-transition elements of p-block show variable oxidation states which differ by a unit of two. For example, tin has oxidation states + 2 and + 4, indium has +1 and + 3, etc.

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On increasing pH (by adding alkali), the H+ ions are used up and according to Le-Chatelier’s principle, the reaction proceeds towards the forward direction producing yellow chromate ions. On decreasing pH (by adding an acid), the reaction shifts towards the backward direction producing the orange dichromate ion.

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Potassium dichromate acts as powerful oxidising agent in acidic medium. In the presence of dil. H₂SO₄, K₂Cr₂O₇ liberates nascent oxygen and, therefore, acts as an oxidising agent.

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Preparation of KMnO4. It is prepared from the mineral pyrolusite (MnO2) by the following steps :
The pyrolusite (MnO2) is fused with caustic potash (KOH) or oxidising agent such as potassium nitrate or potassium chlorate to give a green mass due to the formation of potassium manganate.

The green mass disproportionates in a neutral or acidic solution to give permanganate :

Commercially potassium permanganate is prepared by the alkaline oxidative fusion of MnO₂ followed by electrolytic oxidation of manganate (VI).

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(a) Cr³⁺|Cr²⁺ has a negative reduction potential and, therefore, Cr³⁺ cannot be reduced to Cr²⁺ i.e., Cr³⁺ is most stable.
Mn³⁺|Mn²⁺ has large +ve value and, therefore, Mn³⁺ can be easily reduced i.e., Mn³⁺ is least stable. Fe³⁺|Fe²⁺ has small +ve value and, therefore, Fe³⁺ is more stable than Mn³⁺ but less stable than Cr³⁺.

(b) Reduction potential for Mn²⁺|Mn is most negative and, therefore, it will be most easily oxidized and ease of getting oxidized will be Mn > Cr > Fe.

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1. Electronic configurations. The general electronic configuration of lanthanoids is [Xe]4f^1–14 5d^0–1 6s² whereas that of actionoids is [Rn] 5f^1–14 6d^0–1 7s². Thus, lanthanoids involve the filling of 4f-orbitals whereas actinoids involve the filling of 5f-orbitals.
2. Oxidation states. Lanthanoids have principal oxidation state of +3. In addition, the lanthanoids show limited oxidation states such as +2, +3 and +4 because of large energy gap between 4f and 5d subshells. On the other hand, actinoids show a large number of oxidation states because of small energy gap between 5f and 6d subshells.
3. Atomic and ionic sizes. Both lanthanoids and actinoids show decrease in size of their atoms or ions in +3 oxidation state. In lanthanoids, the decrease is called lanthanoid contraction and in actinoids, it is called actinoid contraction.
4. Chemical reactivity. Lanthanoids are highly electropositive in nature and they have almost similar chemical reactivity. All the actinoids are also electropositive and have high reactivity. However, in general, actinoids are more reactive (especially in finely divided state) than lanthanoids.

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1. E values for Cr³⁺/Cr²⁺ is negative (– 0.41 V) and for Mn³⁺/Mn²⁺ is positive (+1.57V). Thus, Cr²⁺ can undergo oxidation and, therefore, is reducing agent. On the other hand, Mn(III) can undergo reduction, and therefore, acts as an oxidizing agent.
2. In the presence of complexing agents, cobalt gets oxidised from +2 to +3 state because Co (III) is more stable than Co (II).
3. After loss of ns electrons, d¹ electron can easily be lost to give a stable configuration. Therefore, the elements having d¹ configuration are either reducing or undergo disproportionation.

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The disproportionation reactions are those in which the same substance gets oxidised as well as reduced. For example,

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Copper has electronic configuration [Ar] 3d¹⁰ 4s¹. It can easily lose one (4s¹) electron to give stable 3d¹⁰ configuration.

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An alloy is a homogeneous mixture of two or more metals or metals and non-metals.
An important alloy of lanthanoid metals is mischmetal. It contains about 95% lanthanoid metal and about 5% iron and traces of S, C, Ca or Al. It is used in magnesium based alloy to produce bullets, shells and lighter flints.

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The inner transition elements are those in which the last electron enters the f-subshell. These include lanthanoids (from atomic number 58–71) and actinoids (from atomic number 90–103).
Elements having atomic numbers 59, 95 and 102 are inner transition elements.

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The chemistry of actinoids is complicated as compared to lanthanoids because actinoids are radioactive and show wide range of oxidation states.
Lanthanoids show limited number of oxidation states, such as +2, +3 and +4 (+3 is the principal oxidation state). This is because of large energy gap between 5d and 4f subshells. On the other hand, actionoids also show principal oxidation state of +3 but show a number of other oxidation states also. For example, uranium (Z = 92) exhibits oxidation states of +3, +4, + 5, + 6, and + 6 and neptunium (Z = 93) shows oxidation states of + 3, + 4, + 5, + 6 and + 7. This is because of small energy difference between 5f and 6d orbitals.

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Last actinoid is lawrencium (Z = 103)
Electronic configuration : [Rn]⁸⁶ 5f¹⁴ 6d1 7s²
Possible oxidation state : + 3.

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Last actinoid is lawrencium (Z = 103)
Electronic configuration : [Rn]⁸⁶ 5f¹⁴ 6d1 7s²
Possible oxidation state : + 3.

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Ce (Z = 58) : [Xe]⁵⁴ 4f¹ 5d¹ 6s²
Ce³⁺ : 4f¹
No. of unpaired electrons = 1

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1. Z = 61 : [Xe] 4f⁵ 5d⁰ 6s²
2. Z = 91 : [Rn] 5f² 6d¹ 7s²
3. Z = 101 : [Rn] 5f¹³ 6d⁰ 7s²
4. Z = 109 : [Rn] 5f¹⁴ 6d⁷ 7s²

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1. Electronic configurations : In the first transition series, 3d orbitals are progressively filled whereas in second and third transition series 4d and 5d orbitals respectively are filled.
2. Oxidation states : Elements show variable oxidation states in all the transition series. Within a column, the maximum oxidation state increases with atomic number. For example in group 8, iron shows common oxidation states of + 2 and + 3 but ruthenium and osmium in the same group form compounds in + 4, + 6 and + 8 oxidation states.
3. Ionisation enthalpies : The ionisation enthalpies of elements of 5d series are higher than those of 3d and 4d series. This is because in 5d series, there are filled 4f-orbitals. The 4f-orbitals have poor shielding effect. As a result, the outer electrons have greater effective nuclear charge acting on the outermost electrons. Therefore, the ionisation enthalpies are higher.
4. Atomic size : The atomic size of 4d series are larger than those of 3d-series. However, the elements of 5d series have nearly the same size as those of 4d series because of lanthanoid contraction.

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(i) Ti²⁺ : 3d²
For hydrated ions these will be occupied as

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The first transition series possess many properties different from those of heavier elements such as :

1. In first transition series, lower oxidation state is more stable whereas in heavier transition elements higher oxidation states are more stable.
2. The ionisation enthalpy of heavier elements (5d series) is quite higher than those of first transition series.
3. Metal-metal bonding is most common in heavier transition metals but very less in first transition series.
4. The elements of first transition series donot form complexes with higher coordination numbers 7 and 8 which is common with heavier elements.

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