The d and f Block Elements
🔬 Chapter 8: The d and f Block Elements
1. Introduction
The d-block elements (transition elements) are those in which the last electron enters the (n−1)d orbital. The f-block elements (inner transition elements — lanthanoids and actinoids) are those in which the last electron enters the (n−2)f orbital. These elements show characteristic properties like variable oxidation states, colour, catalytic activity, and complex formation.
2. Transition Elements — Definition and Position
Definition (IUPAC): An element whose atom, in its ground state or in any of its common oxidation states, has an incompletely filled d-subshell.
General electronic configuration: (n−1)d¹⁻¹⁰ ns⁰⁻²
Note: Zn, Cd, Hg (Group 12) are NOT considered true transition elements since their d-subshell is completely filled (d¹⁰) in both the elemental and common oxidation states.
3. General Properties of Transition Elements
| Property | Explanation |
|---|---|
| Metallic character | All transition elements are metals — hard, high melting/boiling points, good conductors, due to strong metallic bonding involving d-electrons. |
| Variable oxidation states | Due to the involvement of both ns and (n−1)d electrons with very close energies, allowing several oxidation states to be equally stable. |
| Colour of compounds | Caused by d-d transitions — electrons absorb specific wavelengths of visible light to jump between split d-orbitals (crystal field splitting), and the complementary colour is observed. |
| Catalytic properties | Due to variable oxidation states and ability to form intermediate complexes with reactants, providing an alternate low-energy pathway. E.g. Fe in Haber's process, V₂O₅ in Contact process. |
| Magnetic properties | Most transition metal ions are paramagnetic due to unpaired d-electrons; magnetic moment increases with number of unpaired electrons. |
| Complex formation | Small size, high charge, and availability of vacant d-orbitals allow transition metal ions to accept lone pairs from ligands and form stable complexes. |
| Alloy formation | Similar atomic sizes allow transition metals to easily replace each other in a crystal lattice, forming alloys (e.g. steel, brass). |
| Interstitial compounds | Small atoms like H, C, N can occupy interstitial voids in the transition metal lattice, giving compounds with unusual properties (high melting point, hardness). |
Magnetic moment, μ = √[n(n+2)] BM, where n = number of unpaired electrons
4. Important Compounds of Transition Elements
Potassium Dichromate (K₂Cr₂O₇)
Preparation from chromite ore (FeCr₂O₄):
- Fusion of chromite ore with Na₂CO₃ in presence of air: 4FeCr₂O₄ + 8Na₂CO₃ + 7O₂ → 8Na₂CrO₄ + 2Fe₂O₃ + 8CO₂
- Sodium chromate is acidified to sodium dichromate: 2Na₂CrO₄ + 2H⁺ → Na₂Cr₂O₇ + 2Na⁺ + H₂O
- Treatment with KCl gives orange crystals of K₂Cr₂O₇ (less soluble than NaCl, so crystallises out).
Note: Chromate (CrO₄²⁻, yellow) and dichromate (Cr₂O₇²⁻, orange) interconvert with pH — chromate is stable in alkaline medium, dichromate in acidic medium.
Potassium Permanganate (KMnO₄)
Preparation from pyrolusite ore (MnO₂):
- Fusion with KOH and an oxidising agent (KNO₃ or air): 2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O
- Oxidation (electrolytic or by chlorine) of green manganate to purple permanganate: 3MnO₄²⁻ + 4H⁺ → 2MnO₄⁻ + MnO₂ + 2H₂O
KMnO₄ is a strong oxidising agent, especially in acidic medium, and is used extensively in volumetric (redox) titrations.
5. The Lanthanoids
Elements: Ce to Lu (atomic numbers 58–71), following Lanthanum.
General electronic configuration: [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s²
Lanthanoid contraction: The steady and gradual decrease in atomic/ionic radii of lanthanoid elements with increasing atomic number, due to poor shielding effect of 4f electrons (imperfect shielding of one 4f electron by another).
Consequences of lanthanoid contraction: Causes similarity in properties of elements of second and third transition series in the same group (e.g. Zr and Hf); makes separation of lanthanoids difficult due to their very similar properties.
6. The Actinoids
Elements: Th to Lr (atomic numbers 90–103), following Actinium.
General electronic configuration: [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s²
Key features: All actinoids are radioactive; show a greater range of oxidation states than lanthanoids because 5f, 6d, 7s orbitals are comparable in energy; actinoid contraction is greater per element than lanthanoid contraction due to poorer shielding of 5f electrons.
7. Lanthanoids vs Actinoids — Comparison
| Lanthanoids | Actinoids |
|---|---|
| 4f orbitals are filled | 5f orbitals are filled |
| Mostly non-radioactive (except Pm) | All are radioactive |
| Show limited oxidation states, mainly +3 | Show a wide range of oxidation states |
| Compounds are less basic | Compounds are more basic |
| Poor complex-forming tendency | Greater tendency to form complexes |
8. Important Formula/Concept Summary
| Concept | Key Point |
|---|---|
| d-block config | (n−1)d¹⁻¹⁰ ns⁰⁻² |
| Magnetic moment | μ = √[n(n+2)] BM |
| Chromate ↔ Dichromate | Interconvert with pH (alkaline ↔ acidic) |
| Lanthanoid contraction | Gradual radius decrease due to poor 4f shielding |
| Zn, Cd, Hg | Not true transition elements (d¹⁰ fully filled) |
9. Solved Examples
✏️ Example 1: Calculate the magnetic moment of Mn²⁺ (3d⁵ configuration, 5 unpaired electrons).
Solution: μ = √[n(n+2)] = √[5(5+2)] = √35
μ ≈ 5.92 BM
✏️ Example 2: Why is Zn not regarded as a transition element while Cu is?
Solution: Zn has electronic configuration [Ar]3d¹⁰4s² and forms only Zn²⁺ ([Ar]3d¹⁰) — its d-subshell is completely filled in both the elemental and ionic states, so it fails the IUPAC criterion. Cu has [Ar]3d¹⁰4s¹ but commonly forms Cu²⁺ ([Ar]3d⁹), which has an incompletely filled d-subshell, satisfying the transition element definition.
10. Quick Revision Points
- Transition elements are defined by an incompletely filled d-subshell in the atom OR any common ion.
- Zn, Cd, Hg are NOT transition elements — always d¹⁰, never incompletely filled.
- Colour in transition metal compounds arises from d-d electronic transitions.
- Chromate is stable in alkaline medium (yellow); dichromate is stable in acidic medium (orange).
- Lanthanoid contraction explains the near-identical properties of 4d and 5d elements in the same group.
- Actinoids show a wider range of oxidation states than lanthanoids and are all radioactive.
- Magnetic moment formula μ = √[n(n+2)] BM is a favourite numerical question — always count unpaired electrons carefully from the electronic configuration.
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