The d- and f-Block Elements — NEET Notes

Position in the periodic table — four d-series, two f-series

The d-block sits in the broad middle of the periodic table, spanning groups 3 to 12 and flanked by the s- and p-blocks. Across each long period the (n−1)d orbitals of the penultimate shell are filled progressively, generating four horizontal series: 3d (Sc to Zn), 4d (Y to Cd), 5d (La and Hf to Hg) and 6d (Ac and Rf to Cn). The f-block elements — those in which 4f or 5f orbitals are being filled — are pulled out into the panel at the bottom: the lanthanoids (Ce to Lu) and the actinoids (Th to Lr).

IUPAC's modern definition pins the term transition element to a structural criterion: an element whose neutral atom or any of its common ions has an incomplete d sub-shell. By that yardstick Zn, Cd, Hg (and the synthetic Cn) of group 12 do not qualify — both their ground-state atoms and their common +2 ions have a fully filled d¹⁰ configuration. They are studied alongside the transition metals only because they are the end members of the 3d, 4d and 5d series.

Electronic configuration — (n−1)d¹⁻¹⁰ ns⁰⁻² and two anomalies

The outer electronic configuration of a d-block element is written as (n−1)d¹⁻¹⁰ ns⁰⁻². Here the inner d sub-shell may hold anywhere from one to ten electrons and the outermost s sub-shell holds zero, one or two. The (n−1)d and ns sub-shells lie so close in energy that small effects — exchange stabilisation, symmetrical occupation, electron-pair repulsion — can swap an electron between them. Two such swaps appear in the 3d series and are NEET staples.

The same logic gives palladium (4d¹⁰ 5s⁰), Ag (4d¹⁰ 5s¹), Au (5d¹⁰ 6s¹). Two consequences matter for NEET: when these atoms form ions, ns electrons leave before (n−1)d electrons, so Cr⁰ (3d⁵ 4s¹) → Cr²⁺ is 3d⁴ (not 3d³ 4s¹). And the general formula for Zn, Cd, Hg becomes (n−1)d¹⁰ ns² — completely filled in both atom and common +2 ion, which is why they fail the transition-element test.

Five characteristic properties — the d-electron fingerprint

Partly filled d orbitals are the structural reason behind every property NEET expects you to recite for transition metals. The d orbitals extend further into space than s or p of the same shell, so they interact with surroundings and are themselves perturbed by ligands. NCERT highlights five outcomes that come up again and again.

A sixth, related property is alloy formation. Transition-metal atoms have metallic radii within ~15% of each other, so they form homogeneous solid solutions — chromium, vanadium, tungsten, molybdenum and manganese all alloy with iron to give the steels and stainless steel. Brass (Cu–Zn) and bronze (Cu–Sn) extend the family to non-transition partners.

Variable oxidation states — Mn shows seven

Among transition elements, the elements in or near the middle of each series show the greatest number of oxidation states because they have both unpaired electrons available for sharing and empty orbitals to share into. Manganese exhibits all states from +2 to +7. At the extremes, Sc shows only +3 (too few d-electrons), and Zn only +2 (no d-electron involvement at all). Variable states in the d-block differ from each other by unity, whereas p-block states differ in steps of two — a contrast NEET tests directly.

Down a group, the heavier members favour the higher oxidation state — the opposite of the p-block inert-pair effect. Thus Mo(VI) and W(VI) are more stable than Cr(VI); chromium in dichromate is a strong oxidiser, but MoO₃ and WO₃ are not. The highest oxide of a 3d metal traces the group number up to manganese: Sc₂O₃, TiO₂, V₂O₅, CrO₃, Mn₂O₇ — beyond which the pattern breaks (Fe stops at Fe₂O₃ in stable oxides).

Magnetic moment — the spin-only formula

The d orbital electrons carry a magnetic moment derived from spin angular momentum. For first-row transition-metal complexes the orbital contribution is effectively quenched, so the observed magnetic moment depends only on the count of unpaired electrons. NCERT formalises this as the spin-only formula:

The procedure for any d-block ion: write Z, remove ns electrons before (n−1)d, count the unpaired d-electrons, then plug into μ = √n(n+2). This handles every magnetic-moment PYQ — Co³⁺ (3d⁶, n = 4, μ = √24), Cr³⁺ (3d³, n = 3, μ = √15), Fe³⁺ (3d⁵, n = 5, μ = √35), Ni²⁺ (3d⁸, n = 2, μ = √8) — all tested in NEET 2018 Q.87.

Colour & d–d transitions

Free, gas-phase d orbitals are degenerate — equal in energy. In a complex, the ligands' electric field splits them into two sets separated by an energy gap Δ. An electron from the lower set can absorb a photon of energy hν = Δ — usually in the visible range — and jump to the higher set. The colour seen is the complementary colour of the wavelength absorbed. This is the d–d transition that NCERT lays out in Table 4.8: Ti³⁺ purple (3d¹), V³⁺ green (3d²), Cr³⁺ violet (3d³), Mn²⁺ pink (3d⁵), Fe³⁺ yellow (3d⁵), Cu²⁺ blue (3d⁹).

Ions with no d-electrons (Sc³⁺, Ti⁴⁺, d⁰) or with a fully filled d sub-shell (Zn²⁺, Cu⁺, d¹⁰) cannot undergo d–d transitions and are colourless. NEET 2018 Q.84 hinged on this: of CrO₄²⁻, Cr₂O₇²⁻, MnO₄⁻ and MnO₄²⁻, only MnO₄²⁻ (Mn⁶⁺, 3d¹) shows both d–d transition and paramagnetism — the rest are d⁰. MnO₄⁻ and Cr₂O₇²⁻ are coloured by a different mechanism (charge-transfer, beyond the NEET syllabus), but their d⁰ centres are diamagnetic.

Catalysis, interstitials & alloys

The catalytic prowess of transition metals follows directly from the two features above. Variable oxidation states let the metal accept electrons in one step and donate them in the next, completing the cycle without being consumed. V₂O₅ catalyses the contact process (SO₂ → SO₃), finely divided iron drives Haber's process (N₂ + H₂ → NH₃), and nickel hydrogenates alkenes (the basis of vanaspati). Solid catalysts work because the metal surface forms loose bonds with reactant molecules, raising local concentration and weakening internal bonds — lowering activation energy.

Interstitial compounds arise when small non-metal atoms — H, C, N — slot into the gaps between metal atoms in the crystal lattice. They are non-stoichiometric (TiC, Mn₄N, Fe₃H, VH₀.₅₆, TiH₁.₇), keep the metallic conductivity of the host, and pick up extreme hardness and high melting points. Cementite (Fe₃C) in steel is the most famous example — it is the reason a steel blade outlasts pure iron.

Atomic radii, melting points and lanthanoid contraction

Within a series, the atomic and ionic radii decrease only slightly from left to right because the added d-electron shields the outer ns electrons rather imperfectly — net effective nuclear charge grows, but slowly. Down a group the radius increases from 3d to 4d as expected, but then the 5d radii are nearly identical to the 4d. The cause is the lanthanoid contraction: 14 f-electrons are squeezed in between La and Hf, and 4f shielding is poorer than 3d shielding, so the size shrinks across the whole 4f row and that shrinkage cancels the expected size increase to the 5d series.

Melting points climb sharply across the 3d series, peak at d⁵ (Cr, Mo, W tops the chart with the highest m.p. of any metal at 3683 K), then fall. The cause is metallic bonding: in 3d metals, both 3d and 4s electrons contribute to the lattice; with one unpaired electron per d orbital (d⁵) the bonding electron count is maximised. Mn and Tc are local anomalies — half-filled 3d⁵ 4s² gives stable atoms that bond less strongly than their neighbours. Enthalpies of atomisation track melting points exactly.

Potassium dichromate K₂Cr₂O₇

Potassium dichromate is one of the two inorganic compounds NEET tests at every cycle. Cr is in the +6 oxidation state — its highest. Industrial preparation starts with the chromite ore FeCr₂O₄ and proceeds in three named steps.

The chromate ion CrO₄²⁻ is tetrahedral; the dichromate ion Cr₂O₇²⁻ is two CrO₄ tetrahedra sharing one corner oxygen with a Cr–O–Cr bond angle of 126°. The two ions are interconvertible in solution by pH — acid favours dichromate, alkali favours chromate. The oxidation state of Cr is +6 in both.

In acidic medium, K₂Cr₂O₇ is a powerful oxidant. The half-reaction is the one to memorise:

This drives the standard list of NEET reactions: iodide → iodine, ferrous → ferric (basis of dichrometry), Sn²⁺ → Sn⁴⁺, H₂S → S (precipitate), and acidified K₂Cr₂O₇ with SO₂ gives green Cr₂(SO₄)₃ — the 2016 PYQ. Potassium dichromate is used as a primary standard in volumetric analysis precisely because it is non-hygroscopic and the orange-to-green colour change is unmistakable.

Potassium permanganate KMnO₄

The other compound NEET keeps returning to is potassium permanganate — manganese in its highest state, +7. NCERT's laboratory preparation is a two-step sequence starting from manganese dioxide.

KMnO₄ forms dark purple, almost-black crystals, isostructural with KClO₄. Both manganate (MnO₄²⁻) and permanganate (MnO₄⁻) ions are tetrahedral; π-bonding involves overlap of oxygen 2p orbitals with manganese d orbitals. The green manganate has one unpaired electron (d¹) and is paramagnetic; the permanganate (d⁰) is diamagnetic but intensely coloured by a charge-transfer transition.

In acidic medium permanganate is the strongest of the common oxidants: it converts oxalate to CO₂, Fe²⁺ to Fe³⁺, nitrite to nitrate and iodide to iodine, with manganese reduced all the way to Mn²⁺ (E° = +1.52 V). In neutral or alkaline medium it stops at MnO₂. Permanganate titrations are never done in HCl — HCl would itself be oxidised to chlorine and corrupt the count.

Uses: K₂Cr₂O₇ for leather tanning, primary standard, azo-dye intermediates. KMnO₄ for bleaching wool, cotton, silk; decolouring oils; antiseptic in dilute solution; and as the favourite oxidant of organic chemistry — preparation of carboxylic acids, cleavage of alkenes.

Lanthanoids — the 4f series and lanthanoid contraction

Below the main body of the periodic table sit the fourteen lanthanoids, Ce to Lu, in which the 4f orbitals are filled progressively. Their atomic electronic configurations show 6s² in common, with 4f occupancy variable from 0 (La itself, [Xe] 5d¹ 6s²) to 14 (Lu, [Xe] 4f¹⁴ 5d¹ 6s²). The chemistry of all lanthanoids is overwhelmingly dominated by the +3 oxidation state — every Ln³⁺ ion has a 4fⁿ configuration with n increasing systematically from Ce³⁺ (4f¹) to Lu³⁺ (4f¹⁴).

Occasional +2 and +4 states appear, always near an empty, half-filled or completely filled f sub-shell. Cerium reaches +4 (Ce⁴⁺ = 4f⁰, noble-gas core) and is a strong oxidant; europium drops to +2 (Eu²⁺ = 4f⁷, half-filled, reductant); ytterbium drops to +2 (Yb²⁺ = 4f¹⁴, full, reductant); terbium reaches +4 (Tb⁴⁺ = 4f⁷, half-filled, oxidant). NEET 2016 Q.8 tested precisely this pattern: Gd's stable [Xe] 4f⁷ 5d¹ 6s² preserves the half-filled f, and Eu drops the 5d to keep [Xe] 4f⁷ 6s².

All lanthanoids are silvery-white, soft, reactive metals that tarnish rapidly in air. They burn in O₂ to give Ln₂O₃, react with halogens to give LnX₃, with hydrogen to give hydrides, and with dilute acids to liberate H₂. Their compounds with f-electron counts other than 0 and 14 are paramagnetic. Applications: mischmetal (≈95% Ln + 5% Fe + traces of S, C, Ca, Al) used in lighter flints and Mg-based alloys; mixed Ln oxides as petroleum-cracking catalysts; individual Ln oxides as phosphors in television and fluorescent screens. Cerium(IV) sulphate is a standard analytical oxidant.

Actinoids — radioactive, variable, contracted

The actinoids span Th to Lr — fourteen elements in which 5f orbitals are filled. Every actinoid is radioactive, which is the first feature that complicates their chemistry: experimental work demands shielded facilities and short-lived isotopes that decay before you finish a measurement. Beyond uranium, the elements are entirely synthetic.

The 5f, 6d and 7s orbitals lie much closer in energy than do 4f, 5d and 6s in the lanthanoids. The consequence is the second distinguishing feature: actinoids show a much wider range of oxidation states than lanthanoids — uranium alone exhibits +3, +4, +5 and +6 in stable compounds; neptunium and plutonium reach +7. NEET 2017 asked exactly this and the canonical answer is "5f, 6d and 7s levels have comparable energies." This contrasts sharply with the lanthanoids, where +3 is the only dominant state.

Like the lanthanoids, the actinoids show contraction along the series — the actinoid contraction — and for the same reason (imperfect f–f shielding). Because 5f electrons shield even more poorly than 4f, the actinoid contraction is slightly larger per element than the lanthanoid contraction. Thorium and uranium are excellent fuel sources for nuclear reactors; americium is used in smoke detectors; plutonium and californium are produced for nuclear weapons and neutron-source applications respectively. The actinoids will continue to expand the periodic table — they remain at the frontier of inorganic chemistry.