d- and f-Block Elements
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Direct answer
Transition (d-block) elements have partly filled d orbitals in the atom or its common ions, and that single feature generates their signature properties: variable oxidation states, coloured ions, paramagnetism, catalytic behaviour, interstitial compounds and alloy formation. The exam-favourite compounds are potassium permanganate and potassium dichromate, whose oxidising power depends on the medium. The f-block adds the lanthanoid contraction and its consequences.
What you must remember
- Oxidation states in the 3d series climb to a maximum of +7 at manganese (Sc +3, Ti +4, V +5, Cr +6, Mn +7); +2 grows more stable across the series; zinc shows only +2.
- Colour arises from d-d transitions; magnetism from the spin-only formula mu = sqrt(n(n+2)) BM — Mn^2+ (d5) gives 5.92 BM.
- Stability logic: Cr^2+ is reducing because Cr^3+ (d3) is stable; Mn^3+ is oxidising because Mn^2+ (d5) is; Cu+ disproportionates in water to Cu and Cu^2+.
- Zn, Cd and Hg are d-block but not transition metals — atoms and common ions are d10, so colourless and diamagnetic.
- KMnO4: in acid, MnO4^- + 8H+ + 5e^- → Mn^2+ + 4H2O (purple to colourless); neutral medium gives MnO2 (n-factor 3); strong alkali gives green MnO4^2- (n-factor 1). The purple colour is charge-transfer, not d-d (Mn is +7, d0).
- K2Cr2O7: made from chromite via sodium chromate; in acid, Cr2O7^2- + 14H+ + 6e^- → 2Cr^3+ + 7H2O; yellow chromate and orange dichromate interconvert with pH.
- Lanthanoids are dominantly +3; Ce^4+ is a strong oxidant and Eu^2+ a strong reductant; the contraction from poor 4f shielding makes Zr and Hf nearly identical. Actinoids show wider oxidation states and are all radioactive; mischmetall is a lanthanoid alloy for lighter flints.
- E°(Cu^2+/Cu) is positive: high atomisation and ionisation enthalpies are not offset by hydration — copper cannot displace hydrogen from dilute acids.
Common confusion
The classic confusion is transition metal versus d-block element: zinc sits in the d-block yet fails the "partly filled d" test in both atom and ion, losing the colour and magnetism of true transition metals. Students attribute KMnO4's colour to d-d transitions although manganese is +7 with no d electrons — it is ligand-to-metal charge transfer. In titrations, the medium is the whole story: the same ion takes 5, 3 or 1 electrons per unit in acidic, neutral or strongly alkaline solution.
Exam-focused takeaway
JEE Main tests property statements and single calculations: magnetic moments from unpaired electrons, oxidant behaviour by medium, lanthanoid contraction consequences. JEE Advanced prefers explanation chains — why zinc is not a transition element, why E°(Cu^2+/Cu) is anomalous, why Ce^4+ oxidises, plus the ore-to-salt preparations of KMnO4 and K2Cr2O7. Memorise both oxidants' half-reactions with electron counts; they anchor most numericals.
Frequently asked questions
Why is zinc not a transition element?
Both the atom (3d10 4s2) and Zn^2+ (3d10) have filled d orbitals, so zinc shows no variable valency, coloured ions or paramagnetism.
What is the lanthanoid contraction and what follows from it?
Steady shrinkage of lanthanoid radii from poor 4f shielding; it makes Zr and Hf nearly identical.
Why does KMnO4 behave differently in acidic and alkaline media?
The reduction product changes — colourless Mn^2+ in acid, MnO2 in neutral, green manganate in strong alkali — changing stoichiometry and colour change.
What marks the chromate-dichromate equilibrium?
Yellow chromate turns orange dichromate on acidification and back on alkali: 2CrO4^2- + 2H+ ⇌ Cr2O7^2- + H2O.
Why is KMnO4's colour not a d-d transition?
Manganese is +7 (d0) with no d electron to jump; the intense purple is charge transfer between oxygen and manganese.
Which lanthanoid ions are redox outliers?
Ce^4+ drops to the stable +3, a strong oxidant; Eu^2+ rises to +3, a strong reductant.