The d- and f-Block Elements

2.1 Core concepts

• The d-block contains Groups 3–12 in which (n−1)d orbitals are progressively filled across four long periods, giving four series: 3d (Sc–Zn), 4d (Y–Cd), 5d (La and Hf–Hg) and 6d (Ac and Rf–Cn) (NCERT §Intro/§4.1, p. 89–90).
• IUPAC defines transition metals as metals having an incomplete d subshell either in the neutral atom or in ions; Zn, Cd, Hg (and Cn) have full d¹⁰ configuration in ground state and common oxidation states, so they are not regarded as transition metals though studied with them (NCERT §Intro, p. 89).
• The general outer electronic configuration is (n−1)d¹⁻¹⁰ ns¹⁻², except Pd which is 4d¹⁰5s⁰; small (n−1)d–ns energy gap plus extra stability of half/fully filled sets explains anomalies — Cr is 3d⁵4s¹ (not 3d⁴4s²) and Cu is 3d¹⁰4s¹ (not 3d⁹4s²) (NCERT §4.2, p. 90).
• Almost all transition elements show typical metallic properties — high tensile strength, ductility, malleability, high thermal/electrical conductivity, metallic lustre, hcp/bcc/ccp structures (exceptions Zn, Cd, Hg, Mn) (NCERT §4.3.1, p. 92).
• Melting points are high and rise to a maximum at d⁵ in each row (with anomalies at Mn and Tc); high enthalpies of atomisation arise because one unpaired electron per d orbital favours strong interatomic bonding; 2nd and 3rd series have higher ΔₐH than the first, explaining frequent metal–metal bonding in heavy transition metals (NCERT §4.3.1, p. 92–93).
• Atomic/ionic radii decrease across a series due to ineffective d-electron shielding; 4d radii > 3d radii, but 5d radii are nearly equal to 4d because of lanthanoid contraction — the regular size decrease across the 4f series caused by imperfect shielding of one 4f electron by another (e.g., Zr 160 pm ≈ Hf 159 pm; Nb ≈ Ta; Mo ≈ W) (NCERT §4.3.2, p. 93–94).
• Ionisation enthalpies increase from left to right but more gently than in non-transition elements; irregularities (e.g., Mn⁺, Fe²⁺ stabilities) are explained via exchange energy and the special stability of d⁵ and d¹⁰ configurations. Differences in IE values are also small, so several oxidation states are accessible (NCERT §4.3.3, p. 95–96).
• Transition elements show a great variety of oxidation states differing by unity; Mn shows the widest range from +2 to +7; the highest oxidation states up to Mn equal the sum of s + d electrons (TiO₂ +4, V₂O₅ +5, CrO₄²⁻ +6, MnO₄⁻ +7); +2 is the lowest common state for the 3d series; Sc shows only +3, Zn only +2 (NCERT §4.3.4, Table 4.3, p. 96–97).
• Stability of higher oxidation states is greatest with O and F because they have small size and high electronegativity; fluorine stabilises through high lattice energy/bond enthalpy (CrF₆, VF₅, CoF₃), oxygen also via multiple bonds (Mn₂O₇) and tetrahedral [MO₄]ⁿ⁻ species (NCERT §4.3.7, p. 99–100).
• E°(M²⁺/M) values become less negative across the 3d series due to rising sum of first two ionisation enthalpies; Cu has positive E°(+0.34 V) so Cu cannot liberate H₂ from non-oxidising acids; values for Mn, Ni, Zn deviate due to d⁵ stability (Mn), high hydration enthalpy (Ni) and d¹⁰ (Zn) (NCERT §4.3.5/§4.3.8, p. 98–101).
• Paramagnetism arises from unpaired electrons; for 3d ions orbital contribution is quenched and the spin-only formula μ = √[n(n+2)] BM is used; a single unpaired electron gives 1.73 BM, two gives 2.84 BM, three 3.87, four 4.90, five 5.92 BM (NCERT §4.3.9, p. 101–102).
• Coloured ions arise from d–d transitions: an electron is excited from a lower-energy d orbital to a higher-energy d orbital, the absorbed frequency lies in the visible region, and the observed colour is complementary (e.g., Cu²⁺ d⁹ absorbs in orange-red, looks blue); Sc³⁺ (d⁰) and Zn²⁺ (d¹⁰) are colourless because no d–d transition is possible (NCERT §4.3.10, Table 4.8, p. 103).
• Transition metals readily form complexes due to small size, high charge density and availability of d orbitals; examples include [Fe(CN)₆]³⁻, [Fe(CN)₆]⁴⁻, [Cu(NH₃)₄]²⁺, [PtCl₄]²⁻ (NCERT §4.3.11, p. 103–104).
• Catalytic activity is linked to variable oxidation states and complex-forming ability; V₂O₅ in the Contact Process for SO₃, finely divided Fe in the Haber Process for NH₃, Ni in catalytic hydrogenation of vegetable oils; Fe³⁺ catalyses the reaction of I⁻ with S₂O₈²⁻ by alternating Fe³⁺ ⇌ Fe²⁺ (NCERT §4.3.12, p. 104).
• Interstitial compounds (e.g., TiC, Mn₄N, Fe₃H, VH₀.₅₆, TiH₁.₇) form when small atoms (H, C, N) occupy crystal-lattice voids; they are hard, high-melting, retain metallic conductivity and are chemically inert (NCERT §4.3.13, p. 104).
• Alloys (e.g., ferrous alloys with Cr, V, W, Mo, Mn — stainless steel; brass = Cu–Zn; bronze = Cu–Sn; nichrome = Ni–Cr) form readily because transition metals have similar metallic radii (within ~15%) (NCERT §4.3.14, p. 104).
• K₂Cr₂O₇ is prepared from chromite ore: 4 FeCr₂O₄ + 8 Na₂CO₃ + 7 O₂ → 8 Na₂CrO₄ + 2 Fe₂O₃ + 8 CO₂; sodium chromate is acidified with H₂SO₄ to give dichromate then treated with KCl to crystallise K₂Cr₂O₇ (less soluble than the Na salt). In acidic medium: Cr₂O₇²⁻ + 14 H⁺ + 6 e⁻ → 2 Cr³⁺ + 7 H₂O (E° = 1.33 V); on raising pH chromate forms: Cr₂O₇²⁻ + 2 OH⁻ → 2 CrO₄²⁻ + H₂O. Chromate is tetrahedral; dichromate is two tetrahedra sharing a corner with Cr–O–Cr angle ≈ 126° (NCERT §4.4.1, p. 105–106).
• KMnO₄ is prepared by fusion of MnO₂ with KOH + O₂/KNO₃ giving K₂MnO₄ (green), then electrolytic oxidation (or disproportionation) of MnO₄²⁻ to MnO₄⁻ (purple); commercially MnO₂ → MnO₄²⁻ → MnO₄⁻. KMnO₄ crystals are dark purple, isostructural with KClO₄; on heating: 2 KMnO₄ → K₂MnO₄ + MnO₂ + O₂. MnO₄⁻ is diamagnetic and tetrahedral; manganate (green) is paramagnetic (one unpaired e⁻) (NCERT §4.4.1, p. 106–107).
• Acidified KMnO₄ oxidises I⁻ → I₂, Fe²⁺ → Fe³⁺, C₂O₄²⁻ → CO₂ (at 333 K), H₂S → S, SO₃²⁻ → SO₄²⁻, NO₂⁻ → NO₃⁻; in neutral/faintly alkaline solution it oxidises I⁻ → IO₃⁻, S₂O₃²⁻ → SO₄²⁻ and Mn²⁺ → MnO₂. HCl cannot be used as the medium because it is itself oxidised to Cl₂ — H₂SO₄ is the preferred acid for KMnO₄ titrations (NCERT §4.4.1, p. 107–108).
• Lanthanoids (Ce–Lu, with La included) have common configuration [Xe]4f^n 5d⁰⁻¹ 6s²; Ln³⁺ ions have 4fⁿ; +3 is dominant, with occasional +2 (Eu, Yb, Sm) and +4 (Ce, Pr, Nd, Tb, Dy) — these unusual states are stabilised by empty (Ce⁴⁺ 4f⁰), half-filled (Eu²⁺ 4f⁷) or fully-filled (Yb²⁺ 4f¹⁴) f-subshells (NCERT §4.5.1/§4.5.3, p. 109).
• Lanthanoid contraction (gradual decrease in size across La → Lu, ~10 pm total) is due to imperfect shielding of one 4f electron by another; consequences include nearly equal sizes of 4d/5d transition elements (Zr ≈ Hf, Nb ≈ Ta, Mo ≈ W), the close occurrence and difficult separation of these pairs, and the high density of 5d metals (NCERT §4.5.2, p. 109).
• Lanthanoids are silvery white soft metals, good conductors, with Ln³⁺ E° ≈ −2.2 to −2.4 V (Eu about −2.0 V); they burn in halogens, give Ln₂O₃ and Ln(OH)₃ (basic), and form LnH, Ln₃C/Ln₂C₃/LnC₂, and LnN. Mischmetall (~95% Ln + ~5% Fe + traces of S, C, Ca, Al) is used in Mg-alloy bullets/lighter flints; mixed Ln oxides catalyse petroleum cracking; Ln oxides are TV phosphors (NCERT §4.5.4, p. 110–111).
• Actinoids (Th–Lr, with Ac included) are 5f elements; all are radioactive; configuration is 7s² with variable 5f/6d occupancy. The 5f orbitals are less buried than 4f, so 5f electrons participate in bonding much more — hence a far greater range of oxidation states (up to +7 in Np) (NCERT §4.6, §4.6.1, §4.6.3, p. 111–112).
• Actinoid contraction is greater than lanthanoid contraction owing to poorer shielding by 5f electrons; +3 is the general state but the earlier elements show high states (Th +4; Pa +5; U +6; Np +7); the elements beyond U (Z > 92) are all synthetic (NCERT §4.6.2, §4.6.3, Table 4.11, p. 112).
• Applications: Fe/steel (with Cr, Mn, Ni); TiO₂ in pigments; MnO₂ in dry cells; Zn and Ni/Cd in batteries; V₂O₅ catalyses SO₂ oxidation; TiCl₄/Al(CH₃)₃ are Ziegler catalysts for polymerisation; Fe catalyses Haber process; Ni catalyses fat hydrogenation; PdCl₂ in the Wacker process for acetaldehyde; AgBr in photography (NCERT §4.7, p. 113–114).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Table 4.1 (p. 90–91) — Outer electronic configurations of all four transition series — note Cr (3d⁵4s¹), Cu (3d¹⁰4s¹), Mo (4d⁵5s¹), Ag (4d¹⁰5s¹), Pd (4d¹⁰5s⁰), Pt (5d⁹6s¹), Au (5d¹⁰6s¹) anomalies; understand that half-filled and fully-filled d configurations have extra exchange energy stabilisation.
• Fig. 4.1 (p. 92) — Trends in melting points of 3d/4d/5d series: rises to a peak around d⁵ (Cr 2130 K, Mo 2890 K, W 3680 K), dips at Mn (1517 K, due to weak Mn–Mn bonding from d⁵s² unpaired e⁻ distribution) and Tc; falls steeply at Zn, Cd, Hg (d¹⁰s² — no unpaired d e⁻ for bonding, so Hg is liquid at room T).
• Fig. 4.2 (p. 93) — Enthalpies of atomisation across the series; peak at d⁵ correlates with maximum unpaired electrons available for M–M bonding; second and third series have ΔₐH roughly 1.5–2 times the first series.
• Fig. 4.3 (p. 94) — Trends in atomic radii — 3d < 4d ≈ 5d due to lanthanoid contraction; Zr 160 pm ≈ Hf 159 pm, Nb 146 ≈ Ta 146, Mo 139 ≈ W 139.
• Fig. 4.4 (p. 98) — Observed vs calculated E°(M²⁺/M) for Ti–Zn — deviations at Mn (less negative than expected, because of extra stability of Mn²⁺ d⁵), Ni (more negative than expected, high hydration enthalpy) and Zn (more negative because d¹⁰s² is easy to lose).
• Table 4.7 (p. 102) — Calculated (spin-only) and observed magnetic moments of 3d ions in aqueous solution: Ti³⁺ (d¹) 1.73, V³⁺ (d²) 2.83, Cr³⁺ (d³) 3.87, Mn³⁺ (d⁴) 4.90, Mn²⁺ (d⁵) 5.92, Fe³⁺ (d⁵) 5.92, Fe²⁺ (d⁶) 4.90, Co²⁺ (d⁷) 3.87, Ni²⁺ (d⁸) 2.83, Cu²⁺ (d⁹) 1.73, Zn²⁺ (d¹⁰) 0 BM.
• Table 4.8 (p. 103) — Configurations and colours of 3d M^n+ ions in aqueous solution; Sc³⁺ (d⁰) and Zn²⁺ (d¹⁰) colourless; Ti³⁺ purple, V³⁺ green, Cr³⁺ violet, Mn²⁺ pink, Fe³⁺ yellow, Fe²⁺ green, Co²⁺ pink, Ni²⁺ green, Cu²⁺ blue.
• Fig. 4.6 (p. 109) — Trends in ionic radii of lanthanoids: Ln³⁺ decreases regularly from La³⁺ 103 pm to Lu³⁺ 86 pm — total contraction ~17 pm across 14 elements (~1 pm per element).
• Fig. 4.7 (p. 111) — Chemical reactions of lanthanoids with H₂, halogens, O₂, N₂, C, H₂O, S — all produce expected binary compounds (LnH₂/₃, LnX₃, Ln₂O₃, LnN, Ln₂C₃/LnC₂, Ln(OH)₃ + H₂, Ln₂S₃).
• Structures of CrO₄²⁻ (tetrahedral) and Cr₂O₇²⁻ (two corner-sharing tetrahedra, Cr–O–Cr ≈ 126°) (p. 106) — single Cr⁶⁺ centre with 4 equivalent O in CrO₄²⁻; in Cr₂O₇²⁻, two CrO₃ tetrahedra share one O at the corner; bridging Cr–O bonds are longer than terminal ones.
• Born-Haber-like diagram for KMnO₄ preparation (implicit, p. 106) — fusion step MnO₂ + KOH + O₂ → K₂MnO₄ + H₂O; electrolytic oxidation 2 MnO₄²⁻ → 2 MnO₄⁻ + 2 e⁻ at the anode; the green-to-purple colour change reflects Mn(VI)→Mn(VII).
• Periodic table block diagram (implicit) — shows d-block straddling s and p blocks in Groups 3-12 across periods 4-7, with the f-block (lanthanoids in period 6 row, actinoids in period 7 row) inserted as separate strips below.

2.4 Common confusions / NTA trap points

• Zn, Cd, Hg are in the d-block but are NOT transition elements (full d¹⁰ in atom and common ions); a typical NTA distractor calls them transition.
• Cr is 3d⁵4s¹ and Cu is 3d¹⁰4s¹ — not 3d⁴4s² or 3d⁹4s²; the half-filled/fully-filled d stability is the trap. Similarly Mo (4d⁵5s¹), Ag (4d¹⁰5s¹).
• Spin-only μ requires the count of unpaired electrons in the ION, not the neutral atom. For Fe²⁺ (3d⁶) → 4 unpaired → μ = 4.90 BM, not the atomic count of Fe (4 unpaired, 3d⁶4s² also gives the same in this case but for other ions it differs).
• Permanganate MnO₄⁻ is diamagnetic (Mn in +7 has d⁰); manganate MnO₄²⁻ (Mn in +6, d¹) is paramagnetic — easy confusion.
• KMnO₄ titrations are NOT done in HCl medium because Cl⁻ gets oxidised to Cl₂ — use H₂SO₄ instead. Dilute HNO₃ also works as it does not interfere.
• Lanthanoid contraction is due to poor shielding of 4f electrons (not relativistic effects); the consequence is the near-identity of Zr and Hf radii, NOT a direct decrease in density.
• Cu has a positive E°(M²⁺/M) of +0.34 V; hence Cu does NOT liberate H₂ from dilute HCl/H₂SO₄ (only oxidising acids like HNO₃ or hot conc. H₂SO₄ attack it).
• Cr₂O₇²⁻ in acidic solution is orange and a strong oxidiser; CrO₄²⁻ in alkaline solution is yellow and weakly oxidising — they are interconvertible by pH change without redox.
• The +2 state stability across 3d series increases up to Mn then decreases (Mn²⁺ d⁵ is most stable); above Mn the +3 state becomes harder to achieve.
• Actinoid contraction is GREATER than lanthanoid contraction; common trap is to call them "equal".
• Maximum oxidation state in actinoids is +7 (in Np), not +6 (which is U's well-known state UO₂²⁺); easy MCQ trap.
• For transition metals, ionic radii of M³⁺ < M²⁺ < M, and the spin-only μ ignores orbital contribution (valid for 3d but not as accurate for 4d/5d).