VKC Notes · d-block elements · Geo-Scientist P-I · IFoS P-I
14.3 Transition Elements: 3d, 4d and 5d Compared (Radii, Oxidation States, Redox, Lanthanide Contraction)
★★★ 2021–2025: 8 parts, 65 marks (Geo-Scientist 7 parts / 55 m · IFoS 1 / 10 m). Geo-Scientist asks a d-block “why” question every year: radius trends, why 4d ≈ 5d, high oxidation states, oxidising power.
Syllabus line. Geo-Scientist: “d-block elements: general comparison of 3d, 4d and 5d elements in terms of electronic configuration, elemental forms, metallic nature, atomization energy, oxidation states, redox properties, coordination chemistry, spectral and magnetic properties”. IFoS asks the same lanthanide-contraction pairs under f-block. Splitting of d orbitals is in 13.2 Crystal field theory; the f-block itself in 15.1 Lanthanides and actinides.
Two-page short notes for revisionAll VKC notes
Download PDF PLAN
PDF edition, print-ready. It opens with the VKC plan, together with every locked answer, note and test on the site.
CSE 2024 Q7(a) = asked there; tap to open the solved answerSN = line kept in the short notes
14.3.1 Electronic configurations and the 3d / 4d / 5d comparison ★★★
- General configuration (n−1)d1–10 ns0–2. In the 3d series only Cr 3d⁵4s¹ and Cu 3d¹⁰4s¹ break the pattern (half-filled and filled d). The 4d and 5d series break it more often (Nb 4d⁴5s¹, Mo 4d⁵5s¹, Ru 4d⁷5s¹, Rh 4d⁸5s¹, Pd 4d¹⁰5s⁰, Ag 4d¹⁰5s¹; Pt 5d⁹6s¹, Au 5d¹⁰6s¹) because nd and (n+1)s lie very close in energy.SN
- On ionisation the s electrons leave first: Fe 3d⁶4s² → Fe²⁺ 3d⁶, Fe³⁺ 3d⁵. Every chemical trend below comes from the d electrons being valence electrons that are only partly shielded.
| Property | 3d series | 4d and 5d series |
|---|---|---|
| Radius | smaller (Ti 147 pm) | larger, and 4d ≈ 5d (Zr 160, Hf 159 pm): lanthanide contraction |
| Metallic bonding, ΔₐH, m.p. | weaker; Mn and Zn low | stronger (diffuse d orbitals overlap well); W m.p. 3410 °C; density peaks at Ir (22.65 g cm⁻³) |
| Oxidation states | +2, +3 common; high states strongly oxidising (CrO₄²⁻, MnO₄⁻) | high states stable (MoO₄²⁻, WO₄²⁻, OsO₄, RuO₄, ReF₇); M²⁺(aq) rare |
| M–M bonds, clusters | few | many (Mo₂, Re₂Cl₈²⁻, Nb₆, Mo₆ clusters): 13.5 Metal carbonyls and clusters |
| Coordination number | 4 and 6 | 7, 8, 9 common ([ZrF₇]³⁻, [TaF₈]³⁻, [Mo(CN)₈]⁴⁻) |
| Δ and spin state | high or low spin | Δ larger; almost always low spin |
| Magnetism | spin-only formula works well | strong spin–orbit coupling; μ below spin-only, temperature dependent |
| Noble character | Cu only | Ru, Rh, Pd, Os, Ir, Pt, Ag, Au |
Solved PYQ Geo 2021 Q3(b) 10m
Why are the chemical properties of the d-block elements erratic compared with the uniform chemistry of the lanthanides?
- d orbitals are outer, valence orbitals: they reach the ligands, take part in bonding and are split by the ligand field. Their energies and the number of d electrons change bonding, colour, magnetism and stereochemistry from one element to the next.
- Hence variable oxidation states (Mn +2 to +7), oxidation-state-dependent radii and CFSE-dependent properties (irregular hydration enthalpies, spinel site choices, Jahn–Teller distortions).
- 4f orbitals are inner: buried under 5s²5p⁶, they hardly overlap ligand orbitals; crystal field effects are about 100 cm⁻¹ against 10 000–30 000 cm⁻¹ for d. Bonding is ionic and set by size and charge.
- Lanthanides keep one oxidation state, +3, and their radii fall smoothly (lanthanide contraction), so neighbours differ only slightly in size: uniform chemistry, so similar that ion exchange is needed to separate them (15.1 Lanthanides and actinides).SN
14.3.2 Atomic radii: across a series and down a group ★★★
- Across the 3d series Geo 2023 Q10(a) 10m: Sc 164 → Ti 147 → V 135 → Cr 129 pm, then Mn 137, Fe 126, Co 125, Ni 125, Cu 128, Zn 137 pm.SN
- First, decrease: each step adds a proton and a 3d electron; d electrons shield poorly, so Zeff rises and the atom contracts. More unpaired d electrons also mean stronger metallic bonding, pulling atoms closer.SN
- Then, nearly constant (Fe–Ni): extra d electrons now shield the outer 4s electrons and repel one another, offsetting the nuclear pull; d electrons begin to pair, so metallic bonding stops increasing.SN
- Then, increase (Cu, Zn): the d shell fills (d¹⁰), electrons go into antibonding levels of the band and the d electrons no longer take part in metallic bonding; Zn bonds only through 4s², so it is larger and soft. Mn (137 pm) is an anomaly: half-filled d⁵ is stable and holds its electrons, weakening metallic bonding (and α-Mn has an unusual complex structure).SN
- 3d → 4d: one more shell, so radii increase (Ti 147 → Zr 160 pm).
- 4d → 5d: almost no increase Geo 2022 Q11(a) 10m IFoS 2025 Q8(b)(ii) 10m. Between Zr (Period 5) and Hf (Period 6) Z rises by 32, including the 14 lanthanides that fill 4f. 4f electrons, with their diffuse, many-lobed orbitals, shield very poorly, so Zeff climbs by almost a full unit at each step and the atoms shrink across the lanthanides: the lanthanide contraction. It cancels the growth expected from the extra shell.SN
| Pair | Zr/Hf | Nb/Ta | Mo/W | Ru/Os | Rh/Ir | Pd/Pt |
|---|---|---|---|---|---|---|
| r / pm | 160 / 159 | 147 / 147 | 140 / 141 | 134 / 135 | 134 / 136 | 137 / 139 |
- Consequences: Zr and Hf have nearly identical radii, chemistry and minerals (always found together, hard to separate); 5d metals have high ionisation energies and are more noble; 5d densities nearly double the 4d ones.SN
Draw it in 60 s: three U-shaped lines over groups 3–12; 3d lowest; 4d and 5d drawn on top of each other from group 4 on; label Zr 160 / Hf 159.
Solved PYQ Geo 2024 Q11(c)(i) 5m
Isostructural HfO₂ and ZrO₂ have densities 9.68 and 5.73 g cm⁻³. Explain.
- Because of the lanthanide contraction Hf⁴⁺ and Zr⁴⁺ are the same size, so the two oxides have the same cell volume and Z. Then ρ = ZM/(NAV) ∝ M.
- M(HfO₂) = 178.49 + 2 × 16.00 = 210.49; M(ZrO₂) = 91.22 + 32.00 = 123.22 g mol⁻¹; ratio 1.71. Observed density ratio 9.68 / 5.73 = 1.69.
- Molar volumes M/ρ: HfO₂ 21.7, ZrO₂ 21.5 cm³ mol⁻¹, equal within 1 %: the whole density difference is the mass of Hf.SN
14.3.3 Elemental forms, metallic nature and atomisation energy ★★
- All are metals with hcp, ccp or bcc lattices, mostly hard and high-melting (Hg is a liquid). In the d band, the lower half is net bonding: filling it from group 3 strengthens bonding up to groups 5–6, after which electrons enter the antibonding half and bonding weakens again to group 12.SN
- So enthalpies of atomisation and melting points peak around groups 5–6 (V, Nb, Ta, W) and are lowest at Mn (d⁵ holds its electrons) and Zn, Cd, Hg (d¹⁰, only s electrons bond; Hg is liquid).SN
- 4d and 5d atomisation enthalpies are much larger than 3d: 4d and 5d orbitals are more diffuse and overlap better, giving strong M–M bonds. This is why the heavy metals form clusters and multiple M–M bonds, and why W melts at 3410 °C.SN
Draw it in 60 s: three humped lines over groups 3–12, 5d on top peaking at W, 3d lowest with dips at Mn and Zn; write “4d, 5d ≫ 3d”.
Part of VKC Basic
Read all 10 sections of this topic, its short notes and the PDF with VKC Basic
VKC Basic (₹999 for 6 months) opens every solved answer, the notes, the handbook and the daily answer archive. Every Mains test series includes it.
Solved PYQs, short notes and books: vkcscience.comTelegram: t.me/upsc_chemistry_vkc