Unit 4: d and f Block Elements

Transition metals and inner transition metals — the chemistry of partly filled d and f orbitals

4.1 Electronic Configurations

The d-block occupies groups 3–12, where d orbitals are progressively filled. The f-block (lanthanoids and actinoids) fills 4f and 5f orbitals respectively.

        General configuration: (n-1)d¹⁻¹⁰ ns¹⁻²

        Exceptions: Cr = 3d⁵4s¹ (not 3d⁴4s²), Cu = 3d¹⁰4s¹ (not 3d⁹4s²) — due to extra stability of half-filled and fully-filled d orbitals

Zn, Cd, Hg have d¹⁰ configuration in ground state AND common oxidation states, so they are not technically transition metals by IUPAC definition.

4.2 General Properties and Trends

Physical Properties

Metallic character: High tensile strength, ductility, conductivity
Melting/boiling points: High, peak at d⁵ (strongest metallic bonding from unpaired d electrons)
Atomic radii: Decrease across series (imperfect d shielding), then small increase at end
Density: Increases across series (radius ↓, mass ↑)

Ionization Enthalpies

Gradual increase along series, but less steep than non-transition elements. The 3d electrons shield 4s from increasing nuclear charge. Breaks occur at d⁵ and d¹⁰ configurations.

Electrode Potentials

M²⁺/M values become less negative across the series (general trend). Exceptions: Mn and Zn are more negative than expected (d⁵ and d¹⁰ stability). Cu has positive E° due to high enthalpy of atomization.

4.3 Oxidation States

        Variable oxidation states arise from the small energy difference between (n-1)d and ns orbitals, allowing both to participate in bonding.

        Maximum states near the middle of each series (Mn: +2 to +7). Fewer states at the extremes (Sc, Zn).

Coloured Ions

Transition metal ions are coloured due to d-d transitions. Electrons in lower d orbitals absorb visible light and jump to higher d orbitals. The colour observed is complementary to the absorbed wavelength.

Ion	Config	Colour
Ti³⁺	3d¹	Purple
V³⁺	3d²	Green
Cr³⁺	3d³	Violet
Mn²⁺	3d⁵	Pink
Fe²⁺	3d⁶	Green
Fe³⁺	3d⁵	Yellow
Cu²⁺	3d⁹	Blue

Magnetic Properties

Paramagnetism arises from unpaired d electrons. Magnetic moment is calculated using the spin-only formula:

μ = √(n(n+2)) BM where n = number of unpaired electrons

Catalytic Properties

Transition metals and compounds are excellent catalysts due to their ability to adopt multiple oxidation states and form complexes. Examples: V₂O₅ (Contact process), Fe (Haber's process), Ni (hydrogenation).

4.4 Important Compounds

K₂Cr₂O₇ — Potassium Dichromate

        Preparation: Fusion of chromite ore (FeCr₂O₄) with Na₂CO₃ → Na₂CrO₄ → acidify → Na₂Cr₂O₇ → add KCl → K₂Cr₂O₇ (orange crystals)

        Chromate-Dichromate equilibrium:

        2CrO₄²⁻ + 2H⁺ ⇌ Cr₂O₇²⁻ + H₂O

        (yellow, basic) ⇌ (orange, acidic)

        Oxidizing action (acidic): Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O   (E° = 1.33 V)

KMnO₄ — Potassium Permanganate

        Preparation: MnO₂ fused with KOH + oxidizing agent → K₂MnO₄ (dark green) → disproportionate in acid → KMnO₄ (purple)

        Oxidizing action (acidic): MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O   (E° = +1.51 V)

        Oxidizing power order: VO₂⁺ < Cr₂O₇²⁻ < MnO₄⁻

Other Properties

Interstitial compounds: Small atoms (H, C, N) trapped in metal lattices — very hard, high melting points (e.g., TiC, Fe₃H)
Alloy formation: Transition metals form alloys easily due to similar radii (e.g., stainless steel, brass, bronze)

4.5 Inner Transition Elements (f-Block)

Lanthanoids

Property	Lanthanoids	Actinoids
Series	4f (Ce to Lu)	5f (Th to Lr)
Common oxidation state	+3	+3 to +7 (wider range)
Chemical similarity	Very similar to each other	More complex, varied
Radioactivity	Only Pm is radioactive	All are radioactive
4f/5f shielding	4f shields poorly → contraction	5f shields better than 4f

Lanthanoid Contraction

        Definition: Gradual decrease in atomic/ionic radii from La to Lu due to imperfect shielding of one 4f electron by another.

        Consequences:
        4d and 5d series have nearly identical radii (Zr 160 pm ≈ Hf 159 pm)
Zr and Hf occur together in nature — very difficult to separate
2nd and 3rd transition series elements have very similar properties

Actinoids

Actinoids show more variable oxidation states (+3 to +7). Their chemistry is more complicated due to radioactivity and the wider range of oxidation states. Thorium (Th), Protactinium (Pa), and Uranium (U) are important sources of nuclear energy.

Summary

The d-block elements (transition metals) have partly filled d orbitals and exhibit variable oxidation states, coloured ions, paramagnetism, catalytic activity, and complex formation. Key trends include decreasing atomic radii, increasing ionization energy, and melting points peaking at d⁵. Important compounds include K₂Cr₂O₇ and KMnO₄, both powerful oxidizing agents. The f-block contains lanthanoids (4f) and actinoids (5f). Lanthanoid contraction causes 4d and 5d elements to have nearly identical radii and similar properties.