Unit 4: d- and f-Block Elements

4.1 The Transition Elements (d-Block)

The d-block of the periodic table contains the elements of the groups 3-12 in which the d orbitals are progressively filled in each of the four long periods (3d, 4d, 5d and 6d series). The transition elements are those elements which have incompletely filled d-subshells in their ground state or in any of their common oxidation states. Zinc, cadmium, and mercury of group 12 have full \( d^{10} \) configuration in their ground state as well as in their common oxidation states and hence, are not regarded as transition metals.

General Properties of the Transition Elements:

1. Metallic Character: Nearly all transition metals exhibit typical metallic properties such as high tensile strength, ductility, malleability, high thermal and electrical conductivity, and metallic luster.
2. Melting and Boiling Points: Transition metals have high melting and boiling points due to strong metallic bonding based on the involvement of both ns and (n-1)d electrons.
3. Atomic and Ionic Radii: The atomic and ionic radii of transition metals decrease from group 3 to group 6 due to an increase in effective nuclear charge, then become almost constant, and slightly increase towards the end of the series due to increased electron-electron repulsion.
4. Ionization Enthalpies: Ionization enthalpies generally increase from left to right along a period due to an increase in nuclear charge which accompanies the filling of inner d orbitals.
5. Oxidation States: Transition metals exhibit variable oxidation states due to the participation of both ns and (n-1)d electrons in bonding, as the energy difference between them is small.
6. Coloured Ions: Many transition metal ions are coloured in solid state or in aqueous solution due to d-d transitions. The excitation of an electron from a lower energy d orbital to a higher energy d orbital requires energy which corresponds to a wavelength in the visible region.
7. Catalytic Properties: Transition metals and their compounds are known for their catalytic activity. This is due to their ability to adopt multiple oxidation states and to form complexes. Examples: V2O5 (Contact Process), finely divided iron (Haber’s Process).
8. Magnetic Properties: Due to the presence of unpaired electrons in the (n-1)d orbitals, most transition metal ions are paramagnetic. (\mu = \sqrt{n(n+2)} , \text{BM}) where n is the number of unpaired electrons.
9. Formation of Complex Compounds: Transition metals form a large number of complex compounds due to the comparatively smaller size of the metal ions, high ionic charges, and availability of vacant d orbitals for bond formation.
10. Formation of Interstitial Compounds: Small atoms like H, C, or N are trapped inside the crystal lattices of metals. They are usually non-stoichiometric (e.g., TiC, Mn4N).
11. Alloy Formation: Alloys are formed by transition metals because their atomic radii are very similar. One metal can easily replace another metal in the crystal lattice.

4.2 Preparation and Properties of \(K_2Cr_2O_7\) and \(KMnO_4\)

Potassium dichromate (\( K_2Cr_2O_7 \)):

• Preparation from chromite ore (\( FeCr_2O_4 \)):
  1. Fusion of chromite ore with sodium carbonate in air: \[ 4FeCr_2O_4 + 8Na_2CO_3 + 7O_2 \rightarrow 8Na_2CrO_4 + 2Fe_2O_3 + 8CO_2 \]
  2. Acidification of sodium chromate: \[ 2Na_2CrO_4 + 2H^+ \rightarrow Na_2Cr_2O_7 + 2Na^+ + H_2O \]
  3. Conversion into potassium dichromate: \[ Na_2Cr_2O_7 + 2KCl \rightarrow K_2Cr_2O_7 + 2NaCl \]
• Properties: Strong oxidizing agent in acidic medium. \[ Cr_2O_7^{2-} + 14H^+ + 6e^- \rightarrow 2Cr^{3+} + 7H_2O \quad (E^\circ = +1.33 \text{V}) \]

Potassium permanganate (\( KMnO_4 \)):

• Preparation from pyrolusite ore (\( MnO_2 \)):
  1. Fusion of \( MnO_2 \) with KOH and an oxidizing agent (\( O_2 \) or \( KNO_3 \)): \[ 2MnO_2 + 4KOH + O_2 \rightarrow 2K_2MnO_4 + 2H_2O \]
  2. Electrolytic oxidation in alkaline solution: \[ MnO_4^{2-} \rightarrow MnO_4^- + e^- \]
• Properties: Very strong oxidizing agent.
  • In acidic medium: \( MnO_4^- + 8H^+ + 5e^- \rightarrow Mn^{2+} + 4H_2O \)

4.3 The Inner Transition Elements (f-Block)

Lanthanides

The 14 elements (Ce to Lu) in which the 4f orbitals are progressively filled.

• Oxidation States: The most common oxidation state is +3. Some elements also show +2 and +4 oxidation states in solutions or in solid compounds to attain the stable \( f^0, f^7 \) or \( f^{14} \) configurations.
• Lanthanide Contraction: The steady decrease in the atomic and ionic radii of lanthanide elements with increasing atomic number.
  • Cause: Imperfect shielding of one 4f electron by another in the same subshell. As the nuclear charge increases, the imperfect shielding is unable to counterbalance the effect of increased nuclear charge.
  • Consequences: Similarity in sizes of elements of second and third transition series (e.g., Zr and Hf have almost identical radii). Difficulty in separating lanthanides.

Actinides

The 14 elements (Th to Lr) in which the 5f orbitals are progressively filled.

• They are all radioactive elements.
• Oxidation States: Like lanthanides, the most common oxidation state is +3. However, they exhibit a larger number of oxidation states (+3, +4, +5, +6, +7) because the energy difference between 5f, 6d and 7s orbitals is very small.

Competency-Based Questions (CBQs)

Q1. (CBSE 2023) Explain why zinc, cadmium, and mercury are not regarded as transition metals.

Answer: A transition element is defined as an element having an incompletely filled d-subshell in either its ground state or in any of its common oxidation states. The outermost electronic configuration of:

• Zinc (Zn, Z=30): \( [Ar] 3d^{10} 4s^2 \)
• Cadmium (Cd, Z=48): \( [Kr] 4d^{10} 5s^2 \)
• Mercury (Hg, Z=80): \( [Xe] 4f^{14} 5d^{10} 6s^2 \)

These elements have completely filled d orbitals (\( d^{10} \)) in their ground states as well as in their most stable oxidation states (which is +2 for all three, attained by losing the two s electrons, leaving the \( d^{10} \) core intact). Since they never possess incompletely filled d orbitals, they are not regarded as transition metals.

Q2. (Sample Paper 2024) Account for the following: The metallic radii of the third (5d) series of transition metals are virtually the same as those of the corresponding group members of the second (4d) series.

Answer: This phenomenon is due to the Lanthanide Contraction. In the 6th period, the filling of the 4f subshell occurs before the 5d subshell begins to fill. The 14 elements from Cerium (Z=58) to Lutetium (Z=71) are called lanthanides. The shielding effect of the 4f electrons is very poor. As the atomic number increases across the lanthanide series, the effective nuclear charge increases, pulling the valence shell closer and causing a steady decrease in atomic size. This contraction in size (lanthanide contraction) exactly cancels out the expected increase in size that would normally occur upon moving down the group from the 4d to the 5d series. Therefore, the 5d transition metals (like Hf, Ta, W) have atomic sizes almost equal to their 4d counterparts (like Zr, Nb, Mo).

Q3. (CBSE 2020) A green compound (A) is formed when potassium manganate (\( K_2MnO_4 \)) is prepared by fusing \( MnO_2 \) with KOH in the presence of an oxidizing agent. The aqueous solution of (A) upon electrolytic oxidation yields a deep purple compound (B). 1. Identify A and B. 2. Write the balanced chemical equation for the conversion of A to B in acidic medium by disproportionation.

Answer:

1. The green compound (A) is Potassium manganate (\( K_2MnO_4 \)). The deep purple compound (B) is Potassium permanganate (\( KMnO_4 \)).
2. In a neutral or acidic medium, the green manganate ion (\( MnO_4^{2-} \)) disproportionates to purple permanganate (\( MnO_4^- \)) and solid manganese dioxide (\( MnO_2 \)). Let’s write the balanced equation: \[ 3MnO_4^{2-} (aq) + 4H^+ (aq) \rightarrow 2MnO_4^- (aq) + MnO_2 (s) + 2H_2O(l) \] (Note: 3 moles of manganate(+6) disproportionate to give 2 moles of permanganate(+7) and 1 mole of manganese dioxide(+4).)