d- and f-Block Elements
What this domain tests
Trends across the 3d series (radii, ionisation enthalpy, oxidation states, colour, magnetism, catalytic action, alloy and interstitial compound formation), the lanthanoid contraction and the chemistry of KMnO₄ and K₂Cr₂O₇.
Core rules
- Atomic radii decrease across the series, then rise slightly near the end because of electron–electron repulsion; Mn has an anomalously low ionisation enthalpy.
- Common oxidation states: Sc is only +3; Mn spans +2 to +7; Cr shows +2, +3 and +6; Fe shows +2 and +3.
- Colour arises from d–d transitions. Species with d⁰ or d¹⁰ configurations (Sc³⁺, Ti⁴⁺, Cu⁺, Zn²⁺) are colourless.
- Magnetic moment μ = √(n(n+2)) BM, where n is the number of unpaired electrons.
- Lanthanoid contraction arises from poor shielding by 4f electrons. Consequences include the close similarity of Zr and Hf, decreasing basicity of Ln(OH)₃ across the series, and the stability of Ce⁴⁺ and Eu²⁺.
- KMnO₄: alkaline oxidative fusion of MnO₂ gives K₂MnO₄, which disproportionates in acidic solution. In acid, MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O (n = 5, equivalent weight 31.6); in neutral or basic medium the product is MnO₂ (n = 3, equivalent weight 52.7).
- K₂Cr₂O₇: chromite ore → Na₂CrO₄ → Na₂Cr₂O₇ → K₂Cr₂O₇. In acid, Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O (equivalent weight 49).
Worked example
Fe³⁺ is a d⁵ ion. With five unpaired electrons, μ = √(5 × 7) = √35 = 5.92 BM, matching a high-spin octahedral Fe(III) species. This value is a useful check that the ion is not low spin, which would give one unpaired electron and 1.73 BM.
Traps that cost marks
The equivalent weight of KMnO₄ depends on the medium. Zn, Cd, Hg and Cu(I) (d¹⁰) are not typical transition-metal ions in this context. Do not assign a magnetic moment from the oxidation state alone; count unpaired electrons in the actual crystal-field configuration.