d- and f-Block Elements

Concept overview

The d-block spans groups 3 to 12 — the 'transition metals' with partially filled d-orbitals in their common oxidation states. They share a set of characteristic properties: variable oxidation states, coloured compounds and ions, paramagnetism from unpaired d-electrons, and strong catalytic activity. Iron, copper, zinc, silver and platinum shape modern industry directly through this chemistry.

Variable oxidation states in transition metals arise because the (n−1)d and ns orbitals are close in energy. Manganese ranges from Mn²⁺ to Mn⁷⁺ (in KMnO₄); chromium from Cr²⁺ to Cr⁶⁺ (in K₂Cr₂O₇); iron from Fe²⁺ to Fe³⁺; copper from Cu⁺ to Cu²⁺. The stability of each oxidation state is tied to the resulting electronic configuration — half-filled d⁵ is particularly stable, as is fully-filled d¹⁰.

Colour of transition metal compounds comes from d-d transitions (as discussed in Coordination Compounds) or charge-transfer transitions. Sc³⁺ (d⁰) and Zn²⁺ (d¹⁰) are colourless because they have no d-d transition available; Fe²⁺ and Cu²⁺ are strongly coloured.

Two named reactions dominate JEE Main d-block questions: KMnO₄ chemistry (preparation from pyrolusite, its use in acidic, basic and neutral media, and the specific products in each) and K₂Cr₂O₇ chemistry (preparation from chromite, its conversion to K₂CrO₄ in alkali). Learn the products in each medium and drill 15 problems.

The f-block splits into lanthanides (4f, Ce to Lu) and actinides (5f, Th to Lr). Lanthanide contraction — the gradual decrease in ionic radii from La³⁺ to Lu³⁺ — has far-reaching consequences: it makes second-row and third-row transition metals of the same group nearly identical in size (Zr and Hf, Nb and Ta, Mo and W), which is why they're so hard to separate chemically. Actinides are all radioactive; only thorium and uranium occur naturally in appreciable quantities.

Key formulas & concepts

• Transition metal characteristics: Variable oxidation states, colour, magnetism, catalysis, alloy formation
• Highest ox state (Mn): +7 in MnO₄⁻ (permanganate)
• Highest ox state (Cr): +6 in CrO₄²⁻ (chromate) and Cr₂O₇²⁻ (dichromate)
• KMnO₄ in acidic medium: MnO₄⁻ → Mn²⁺ (5 e⁻ change; purple → colourless)
• KMnO₄ in basic medium: MnO₄⁻ → MnO₂ (3 e⁻ change; purple → brown)
• K₂Cr₂O₇ in acidic medium: Cr₂O₇²⁻ → 2Cr³⁺ (6 e⁻ change; orange → green)
• Lanthanide contraction: Ionic radius decreases from La³⁺ (103 pm) to Lu³⁺ (85 pm)

Common mistakes to avoid

• Predicting Zn as a typical transition metal — Zn is d¹⁰ in its common +2 state and does not show variable oxidation, colour or paramagnetism, so it is not strictly a transition metal.
• Assuming KMnO₄ gives Mn²⁺ in all media — only in acidic; in basic it gives MnO₂ (brown), in neutral it gives MnO₂ + OH⁻.
• Forgetting that lanthanide contraction is why Zr and Hf are so similar — this makes their separation industrially painful.
• Confusing d⁵ half-filled stability with fully-filled d¹⁰ — both are stable, but for different reasons (exchange energy vs full-shell stability).

JEE Main relevance

JEE Main asks 2 d-block questions every paper — one on KMnO₄/K₂Cr₂O₇ chemistry (equation-balancing or product identification) and one general transition-metal-property question. f-block appears less often but reliably tests lanthanide contraction.

JEE Advanced relevance

Advanced problems combine transition-metal chemistry with coordination — for instance, asking whether a specific complex is a mineral acid oxidant or a mild oxidant based on its E° value, or predicting the magnetic moment of a specific transition-metal complex.

Previous-year questions (PYQ)

d-block PYQs cluster around KMnO₄ / K₂Cr₂O₇ chemistry. This is a chapter where 15 questions of focused practice will yield a reliable mark on paper day.