# d- and f-Block Elements

> Transition metals, oxidation states, colour, magnetic moment, lanthanoid contraction, KMnO4 and K2Cr2O7 — NEET-UG Chemistry, NCERT Class 12.

- Canonical URL: https://prepelephant.com/topics/neet-ug/chemistry/d-and-f-block-ncert
- Exam / course: NEET-UG · Subject: Chemistry
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "d- and f-Block Elements", PrepElephant, https://prepelephant.com/topics/neet-ug/chemistry/d-and-f-block-ncert

## Direct answer

A partly filled d subshell gives the transition metals their identity — variable oxidation states (manganese spans +2 to +7), colour from d–d transitions, paramagnetism counted as √n(n+2) Bohr magnetons, and catalytic power from surface adsorption and variable valence. Zinc, cadmium and mercury own full d10 configurations, so despite living in the d block they are not transition elements. Among the f-block lanthanoids, the poor shielding of 4f electrons contracts the series, making zirconium and hafnium chemical twins, while permanganate and dichromate — with E° values of 1.51 V and 1.33 V in acidic solution — supply the block's most examinable oxidants.

## What you must remember

- Mn shows the maximum oxidation state (+7) in the 3d series, matching its 3d5 4s2 electrons; Sc is the only one restricted to +3.
- Colour needs partially filled d orbitals: Cu2+ (d9) is blue, Cu+ (d10) colourless; Ti3+ (d1) purple.
- Spin-only magnetic moment μ = √n(n+2) BM: Fe3+ (d5) gives 5.92 BM, Mn2+ the same, Cr3+ (d3) 3.87 BM.
- Standard potentials: MnO4− + 8H+ + 5e− → Mn2+ + 4H2O, E° = 1.51 V; Cr2O7^2− + 14H+ + 6e− → 2Cr3+ + 7H2O, E° = 1.33 V.
- KMnO4 titrates Fe2+, oxalate, I− and H2S in acid, and turns MnO2 in neutral or faintly alkaline medium; alkaline KMnO4 is Baeyer's reagent for unsaturation.
- Chromate (yellow) and dichromate (orange) interconvert with pH: 2CrO4^2− + 2H+ ⇌ Cr2O7^2− + H2O.
- KMnO4's purple is a charge-transfer colour, not d–d — Mn(VII) has no d electrons at all.
- Mischmetall (about 95 per cent lanthanoid + 5 per cent iron) goes into lighter flints and bullet alloys.

## Counting unpaired electrons: Fe3+ versus Fe2+

Calculate the magnetic moment of Fe3+. Start from the electron count: iron is [Ar]3d6 4s2, and Fe3+ loses the two 4s electrons plus one 3d electron, leaving [Ar]3d5 — five unpaired electrons by Hund's rule. Substitute into the spin-only formula: μ = √(n(n+2)) = √(5 × 7) = √35 = 5.92 BM. Compare Fe2+, which is [Ar]3d6 with four unpaired electrons: μ = √(4 × 6) = √24 = 4.90 BM. The diagnostic power runs backwards too — measure 5.92 BM, count five unpaired electrons, identify a d5 ion; that is how examiners connect magnetism to configuration in a single question. Two cautions keep it honest: the formula is spin-only, ignoring orbital contributions, and it presumes mononuclear complexes where pairing obeys the free-ion pattern. For the same reason Zn2+ (d10) is diamagnetic with μ = 0, a boundary case that appears as a distracter in almost every magnetic-moment question set.

## The d-block distracters

Calling Zn, Cd and Hg transition elements is the oldest trap in the chapter: a transition element needs a partly filled d subshell in the element or a common ion, and d10 in both disqualifies them. Second, the permanganate colour is not d–d — with Mn at +7 there are no d electrons; the intense purple comes from ligand-to-metal charge transfer, and the same logic explains chromate's yellow. Third, the lanthanoid contraction needs both cause (poor 4f shielding) and consequence (Zr and Hf nearly identical radii and chemistry, so they occur together and are hard to separate) — half-remembered answers lose the mark. Fourth, the chromate–dichromate equilibrium is pH-driven both ways: acid pushes it orange, alkali pushes it back yellow. Finally, KMnO4's medium decides the product — Mn2+ in acid, MnO2 in neutral — and titration questions live on exactly that distinction.

## Frequently asked questions

### Why are Zn, Cd and Hg not transition elements?

Their atoms and common ions all have filled d10 configurations, so they lack the partially filled d subshell that defines transition character and its properties.

### Why is KMnO4 coloured although Mn(VII) has no d electron?

The colour arises from ligand-to-metal charge transfer (oxygen to manganese), not from d–d transitions which are impossible without d electrons.

### What is the spin-only magnetic moment of Fe3+?

5.92 BM, from five unpaired electrons in the 3d5 configuration — μ = √(5 × 7) BM.

### What causes the lanthanoid contraction?

The 4f electrons shield the nuclear charge poorly, so the effective nuclear charge felt by outer electrons increases steadily across the series and radii shrink.

### What does KMnO4 give on reduction in neutral medium?

Brown-black MnO2, unlike the nearly colourless Mn2+ produced in acidic solution.