d & f block elements (Chapter 8) class XII

d- and f- block elements

d-block elements represent change in properties from most electropositive s-block elements to least electropositive p-block elements. So, these are called transition metals.

Transition elements are those elements which have partially filled d-subshells in their elementary state or in their commonly occurring oxidation states.

General configuration = (n-1) d^1-10 ns^1-2

There are four transition series:

3d – Sc        Ti         V        Cr         Mn           Fe         Co        Ni      Cu       Zn

4d – Y          Zr        Nb     Mo        Tc            Ru          Rh       Pd      Ag      Cd

5d – La        Hf        Ta       W         Re           Os            Ir        Pt       Au      Hg

6d – Ac        Ku       Ha

Cu = 3d¹⁰ 4s¹. But in its most common oxidation state (+2), it contains one unpaired electron in its d-orbital. So, it is a transition element. Cu²⁺ = 3d⁹ 4s⁰. Similarly, Ag and Au are also transition elements.

Physical Properties –

1) Atomic radii – Across the period (series), atomic radii of elements of a particular series decrease with increase in atomic number and then  become constant in mid-way and at last, there is an increase in atomic radius.

Reason – With increase in atomic number, nuclear charge goes on increasing and force of attraction between nucleus and valence electrons increases due to which atomic radius decreases with increase in atomic number in the beginning.

But as the electrons are increased in d-orbitals. These electrons screen the outermost s-electrons. So, as the d-electrons increase, screening effect increases which neutralises the effect of increased nuclear charge and hence, atomic radius remains almost unchanged after chromium.

At the end of the series, there is slight increase in the atomic radii due to increased electron-electron repulsions between electrons in same orbitals due to which electron cloud expands and size increases.

Down the group, atomic radii increase due to increase in number of shells. But atomic radii of elements of 4d-series and 5d-series are nearly same due to Lanthanide Contraction.

2) Metallic character - All transition elements are metals. They have high density, hardness, high melting point and boiling point, malleable, ductile, conduct electricity and heat.

Reason – Metallic character is due to their low ionization energies and number of vacant orbitals in outermost shell. Greater the number of unpaired d-electrons, greater is the number of bonds and greater is the strength of metallic bonds. E.g. Cr, Mo and W (tungsten) have maximum number of unpaired electrons and are very hard metals.

Zn, Cd and Hg do not have any unpaired electrons. So, they are not hard (nd¹⁰). They are also known as non-transition metals.

3) Density – All metals have high density. Osmium has the highest density.

4) Melting point and boiling point – They have high melting point and boiling point due to strong metallic bonds between atoms of elements.

Metallic bond is formed due to the interaction of electrons in outermost orbitals. Greater the number of valence electrons, stronger is the metallic bonding and higher is the melting point. In a particular series, metallic strength increases upto middle with increasing availability of unpaired electrons upto d⁵ configuration (Sc =1, Ti =2, V =3, Cr =4, Mn =5) and then decreases with decreasing availability of unpaired electrons in d-orbital. Therefore, melting point decreases.

5) Ionization enthalpy – Energy required to remove an electron from valence shell of an isolated gaseous atom. First ionization enthalpy is higher than those of s-block elements and lesser than p-block elements.

Reason – Due to increase in nuclear charge and force of attraction between nucleus and valence electrons. Hence more energy is required to remove an electron from outermost shell.

Also, the effect of increasing nuclear charge is opposed by additional screening effect of nucleus and hence, ionization enthalpy increases but quite slowly among d-block series.

First ionization enthalpy of 5-d series is higher than those of 3d- and 4d- series due to poor shielding effect of 4f-orbitals. As a result, outer electrons have greater nuclear charge acting on outer valence electrons.

Consequences: a) Ni (II) compounds are thermodynamically more stable than Pt(II) compounds. Pt(IV) compounds are more stable than Ni(IV) compounds. E.g. K₂PtCl₆ exist but not K₂NiCl₆.

6) Oxidation state – Transition metals exhibit variable oxidation states.

Reason- This is due to the participation of inner (n-1) d-electrons in addition to outer n electrons because energies of ns- and (n-1) d subshells are almost equal. They contain many unpaired electrons.

Highest oxidation state is shown by Osmium(Os) and Rhuthenium(Ru) of +8.

Few transition metals also form compounds in low oxidation states like +1, 0, -1. E.g. [Ni(CO)₄], [Fe(CO)₅] in which Ni and Fe have zero oxidation states.

7) Formation of coloured ions – s and p-block elements are white in colour while d-block elements are coloured in solid or solution form.

Reason- It occurs due to the presence of unpaired electrons. The energies of five d-orbitals in the same subshell do not remain equal during the formation of complexes or compounds of transition metals. Under the influence of approaching ions towards the central metal ion, d-orbitals split into different energy levels. This phenomenon is known as crystal field splitting (CFS).

In case of transition metal ions, electrons are excited from one energy level to another in same d-subshell. These are known as d-d transitions. The amount of energy required to excite unpaired electrons to higher excited states within the same d-subshell corresponds to energy of certain colours of visible light and the observed colour of substance is always complementary colour of colour absorbed by the substance.

Zn, Cd and Hg compounds are generally white in colour because of completely filled d-orbitals i.e. they do not have any unpaired electron (d¹⁰).

8) Magnetic properties – Most of the compounds of transition elements are paramagnetic due to the presence of unpaired electrons in the d-orbitals of metal ion. Unit of magnetic moment (μ) = Bohr Magneton.              μ =    n ( n + 2 )        where n = number of unpaired electrons

Iron oxide = Ferromagnetic substance (highly magnetic).

Zn²⁺, Cd²⁺, Hg²⁺, Cu⁺ compounds = Diamagnetic (no unpaired electron).

9) Tendency to form complexes – Transition elements form large number of coordination compounds. The transition metal ions bind to number of anions or neutral molecules (ligands) in the complexes like [Ni(NH₃)₆]²⁺, [Co(NH₃)₆]³⁺, etc.

Reason- due to small size of transition metal atoms or ions, high nuclear charge, presence of vacant d-orbitals of suitable energy are available to accept lone pair of electrons donated by ligands.

10) Formation of interstitial compounds – Transition metals form interstitial compounds with elements like B, C and N i.e. they get trapped in vacant spaces of lattices of these metals.

Reason-  Transition metals can easily accommodate the vacant spaces of these non-metals due to their small size, defects in structures and their variable oxidation states.

11) Catalytic properties – Many transition metals and their compounds act as good catalyst for many reactions like Ni, Pt, Pd, V₂O₅, MnO₂, etc.

Reason- (i) due to the presence of vacant orbitals and their tendency to show variable oxidation states, (ii) Transition metals provide large surface area on which reactants may be absorbed for the reaction to occur. E.g. in Contact process, solid V_2­O₅ adsorbs SO₂ for reaction.

12) Alloy formation - Transition metals form large number of alloys.

Reason- Transition metals are quite similar in size and atoms of one element in its crystal lattice substitute atoms of other metal in its crystal lattice. E.g. Brass = Cu + Zn, Bronze = Cu + Sn, Stainless steel = Fe + Cr + Ni (high melting point, hard, more resistant to corrosion).

13) First row transition metals – Oxides- General formulae: MO, M₂O₃, M₃O₄, MO₂, M₂O₅, MO₃ .

Acidic character increases as oxidation state increases.

Oxide :                   MnO        Mn₂O₃          Mn₃O₄          MnO₂       Mn₂O₇

Oxidation:               +2               +3               +8/3               +4             +7

Reducing character of oxides –Across the period, reducing character first increases then decreases as oxidation state. Lower oxidation state is more stable in 3d-series i.e. +2 state. V₂O₅ and CrO are strong reducing agents.

 As we move down the group, higher oxidation states become more stable. E.g. Osmium(VIII) oxide (OsO₄) is known but iron (VIII) oxide is not known. Similarly, hexafluorides of 4d- and 5d-series exist.

K₂Cr₂O₇  (potassium dichromate):

Preparation: from chromite ore

(i) 4FeCr_2­O₄ + 16NaOH + 7O₂ → 8Na₂CrO₄ + 2 Fe₂O₃ + 8H₂O

(ii) 2Na₂CrO₄ + H₂SO₄ → Na₂Cr₂O₇  + Na₂SO₄ + H₂O.  Na₂SO₄ can be removed by crystallization.

(iii) Na₂Cr₂O₇  + 2KCl → K₂Cr₂O₇  + 2NaCl. So, orange crystals of K₂Cr₂O₇  are obtained.

Properties: (i) It is orange red crystalline solid, soluble in water.

(ii) Action of heat: 4 K₂Cr₂O₇     →  4 K₂Cr₂O₄ + 2 Cr₂O₃ + 3O₂

(iii) K₂Cr₂O₇  + 2 KOH → 2 K₂Cr₂O₄ + H₂O

2 K₂CrO₄ + H₂SO₄ → K₂Cr₂O₇  + K₂SO₄ + H₂O

(iv) Chromyl chloride test – used to test the presence of chloride ions.

K₂Cr₂O₇ + 4NaCl + 6H₂SO₄ → 2KHSO₄ + 4NaHSO₄ + 3H₂O + 2Cr₂O₂Cl₂ (chromyl chloride red vapours)

(v) K₂Cr₂O₇ + 4HCl → 2KCl + 2CrCl₃ + 7H₂O + 3Cl₂

(vi) Oxidising character- K₂Cr₂O₇ is a powerful oxidizing agent.

K₂Cr₂O₇ + 4H₂SO₄ → K₂SO₄ + Cr₂(SO₄)_­3­ + 4H₂O + 3[O]

Strong oxidizing agent in acidic medium:

Cr₂O₇^2- + 14 H⁺ + 6e^- → 2Cr³⁺ (green) + 7 H₂O

Uses: a) in volumetric analysis of Fe²⁺, I^-.    b) in photography for hardening gelatin film.  c) in dyeing.

Note: a) 2I^- → I₂ + 2e^-           b) Fe²⁺ → Fe³⁺  + e^-                 c) H₂S → S + 2H⁺ + 2e^-

d) H₂C₂O₄ → 2CO₂ + 2H⁺ + 2e^-                e) SO₃^2- + H₂O  → SO₄^2- + 2e^- + 2H⁺ 

KMnO₄ = Potassium permanganate

Preparation: 1) a) 2MnO₂ + 4KOH + O₂ →2K₂MnO₄ + 2H₂O

b) 3MnO₄^2- + 4H⁺ → 2MnO₄^- + MnO₂ + 2H₂O   [Disproportionation reaction]

2) Commercial process:  KMnO₄ is prepared by the alkaline oxidative fusion of MnO₂ followed by electrolytic oxidation of manganite (VI)

Properties: a) It is dark violet crystalline solid.

b) It is fairly soluble in water.

c) 2KMnO₄                K₂MnO₄ + MnO₂ + O₂

d) 4KMnO₄ + 4KOH → 4K₂MnO₄ + 2H₂O + O₂

e) It is powerful oxidising agent in neutral, alkaline or acidic solutions. MnO₄^-  + e^-  → MnO₄^2-_,

MnO₄^- + 4H⁺ +3e^-  → MnO₂ + 2H₂O,                MnO₄^- + 8H⁺ +5e^-  → Mn²⁺ + 4H₂O.

Uses: as an oxidizing agent in labs and industries, used as disinfectant for water, in quantitative and qualitative analysis.

E⁰ values (standard electrode potential) become less negative across the series is related to the general increase in sum of the first and second ionization enthalpies.

More the ionization enthalpy is, more is the reduction potential and lesser is the oxidation potential. So, E⁰ value (reduction potential) becomes more negative.

Exception: a) E⁰ for Mn, Ni and more (-) than expected from the trend. This is due to the stability of the half-filled d subshell in Mn²⁺ and completely filled d¹⁰ configuration in Zn²⁺, whereas E⁰ for Ni is related to the highest negative Δ_hydH⁰ (hydration energy).

b) Cu²⁺ = 3d⁹ 4s⁰,   Cu⁺ ₌ 3d¹⁰ 4s⁰

Cu²⁺ is exceptionally more stable than Cu⁺ in aqueous solutions because of more negative Δ_hydH⁰ of Cu²⁺ (aq) than Cu⁺ , which is more than compensates for the second ionization enthalpy of Cu. So, Cu⁺ undergoes disproportionation in aqueous solution. 2 Cu⁺ (aq) → Cu²⁺ + Cu. E⁰ = + 0.34V (favourable E⁰ value).

c) All Cu²⁺ halides are known except the iodide. In this case, Cu²⁺ oxidises I^-  to I₂.

2 Cu²⁺ + 4 I^- → Cu₂I₂ (s) + I₂

Stable d-configurations – d³ = stable half-filled t_2g configuration, d⁵ = half-filled d-orbital, d⁶ = fully-filled t_2g level, d¹⁰ = fully-filled d-orbital.

f-block elements: Elements in which last electron enters the f-orbital of their atoms are called f-block elements. They are known as inner transition elements.        

 General Configuration: (n-2)f^1-14 (n-1)d^0-10 ns^1-2      (n-2) = Anti-penultimate shell,   (n-1) = Penultimate shell,   n = main ( valence ) shell.

They consist of two series of elements placed at the bottom of periodic table by filling of electrons in 4f- and 5f-orbitals.

(i) Lanthanoids or lanthanides-  The series involving the filling of 4f-orbitals following Lanthanum (Z=57), Cerium (Z=58) to Lutetium (Z=71).

They occur very rarely and are known as rare earth metals.

Z=57 = La (Lanthanum) = 5d¹ 6s²,    Z=58 = Ce (Cerium) = 4f¹  5d¹ 6s²,     Z=59 = Pr (Praseodymium) = 4f²  6s²,      Z=63 = Eu (Europium) = 4f⁷ 6s²     ,        Z=64 = Gd  (Gadolinium) = 4f⁷  5d¹ 6s²,                            Z=71 = Lu (Lutetium) = 4f¹⁴  5d¹ 6s².

(ii) Actinides - The series involving the filling of 5f-orbitals following Actinium (Z=89), Thorium (Z=90) to Lawrencium (Z=103).

Actinium (Z=89) = Ac = 6d¹ 7s²,  Thorium (Z=90) = Th = 6d² 7s², Curium (Z=96) = Cm = 5f⁷ 6d¹ 7s², Lawrencium (Z=103) = Lr = 5f¹⁴ 6d¹ 7s²

The Lanthanoids- All Lanthanoids have common oxidation state of +3 with +2 and +4 also by attaining stable f⁰, f⁷, f¹⁴ configurations.

Physical Properties: All lanthanides are soft, malleable, and ductile with low tensile strength, bad conductors of heat and electricity.

General Charcteristics:  1) Trivalent ions of lanthanides are coloured in solid state as well as in solution due to the absorption in visible light of spectrum by unpaired electrons resulting in f-f transitions.

2) They are paramagnetic. All lanthanides are highly electropositive metals and have almost similar reactivity due to the fact that lanthanides differ only in number of 4f electrons. Since these electrons are very effectively shielded from interaction by 6s, 5p electrons, hence they show little differences in their chemical reactivity. So, they are difficult to separate.

f⁰ and f¹⁴ are colourless and diamagnetic [La³⁺ and Lu³⁺]

3) Atomic radii- The steady decrease in atomic sizes and ionic sizes of lanthanides with increasing atomic number is known as lanthanide contraction.

Reason: As atomic number increases across the series, nuclear charge increases by one unit. The new electrons are added to same inner 4f-subshells. But 4f electrons shield each other from nuclear charge quite poorly because of very diffused shapes of f-orbitals.

Consequences of lanthanide contraction:  

a) Elements of 4f- and 5f- transition series resemble each other due to similar sizes.

b) It is difficult to separate these elements in pure state. Fractional crystallization method is done repeatedly to separate them.

c) Due to lanthanide contraction, size of lanthanide ions decrease with increase in atomic number due to which the covalent character between Lanthanide ion and OH^- ions increase from La³⁺ to Lu³⁺. Therefore, basic strength of hydroxides decreases with increase in atomic number across the series. Thus, La(OH)₃ is most basic and Lu(OH)₃ is least basic.

The actinoids-

General Characteristics: 1) These metals are all silvery in appearance.

2)Antinoid contraction- The steady decrease in atomic sizes and ionic sizes of antinoids with increasing atomic number is known as actinide contraction.

Reason: As atomic number increases across the series, nuclear charge increases by one unit. The new electrons are added to same inner 5f-subshells. But 5f electrons shield each other from nuclear charge quite poorly.

3) They are radioactive elements and earlier members have relatively long half lives, the latter ones have half lives ranging from a day to 3 minutes for Lawrencium (Z=103). The latter members are prepared only in nanogram quantities. These facts render their study more difficult.

Lanthanoids	Actinoids
1) Tendency to form complexes is less.	1) More tendency to form complexes.
2) Non-radioactive except promethium.	2) All are radioactive.
3) All show +3 oxidation state and few show +2 and +4 also.	3) All show +3, +4, +5, +6 and +7 oxidation state.
4) They are less basic.	4) They are more basic.

Misch Metall- An alloy of a lanthanoid metal (95%) and iron (5%) with traces of S, C, Ca and Al. It is used in Mg-based alloy to produce bullets, shell and lighter flint.

Uses of Lanthanoids: (i) For production of alloy steels for plates and pipes. (ii) Mixed oxides of lanthanoids are used as catalysts in petroleum cracking.

Reactions of Lanthanoids:

2Ln + 3X₂ → 2LnX₃ ,        2Ln + 6H₂O →  2Ln(OH)₃ + 3H₂ ,

Ln + O₂ → Ln₂O₃ ,              Ln + S →Ln₂S₃ ,                Ln + N₂ → LnN,                         Ln + C  →  LnC₂