f-Block

f-Block

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

• f-block elements are those whose last electron enters the (n−2)f orbital, placing them as inner transition metals below the periodic table: lanthanoids (4f, Z = 58–71) and actinoids (5f, Z = 90–103).
• General configuration: (n−2)f¹⁻¹⁴ (n−1)d⁰⁻¹ ns². Ce has 4f¹ 5d¹ 6s²; Gd has 4f⁷ 5d¹ 6s²; Lu closes with 4f¹⁴ 5d¹ 6s².
• Lanthanoid contraction: steady decrease in Ln³⁺ radii from La³⁺ to Lu³⁺ because 4f electrons shield poorly. The same effect, larger in magnitude, is seen across the actinoids.
• Common oxidation state is +3 for both series; Ce⁴⁺ (f⁰) and Eu²⁺, Yb²⁺ (f¹⁴) are the well-known exceptions in lanthanoids.
• Spin-only magnetic moment: μ = √(n(n+2)) BM, where n = number of unpaired electrons — applied to Gd³⁺ gives √63 ≈ 7.94 BM.
• CUET pointer: expect 1–2 questions — usually on lanthanoid contraction consequences, electronic configuration of Ce/Lu/Gd, or identifying the most stable oxidation state.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Electronic Configuration

Lanthanoids build the 4f sub-shell from Ce (Z = 58) to Lu (Z = 71). The standard pattern is [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s². Exceptions worth memorising for CUET:

Element	Configuration	f-electrons
Ce (58)	[Xe] 4f¹ 5d¹ 6s²	1
Gd (64)	[Xe] 4f⁷ 5d¹ 6s²	7 (half-filled stable)
Lu (71)	[Xe] 4f¹⁴ 5d¹ 6s²	14 (fully filled stable)

Actinoids follow [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s², with Th, Pa, U, Np showing irregular 6d occupancy because 5f and 6d energies are nearly equal.

Lanthanoid Contraction — Origin and Consequences

Because 4f orbitals are diffuse and sit inside the (n−1)d and ns shells, each added 4f electron screens the nuclear charge poorly. Effective nuclear charge felt by outer electrons therefore rises gradually, pulling radii inward. Across the series, ionic radii shrink by ~11 pm (La³⁺ ≈ 103 pm → Lu³⁺ ≈ 86 pm).

Direct consequences tested in CUET:

• Zr (4d) and Hf (5d) have nearly identical radii (~155 pm), making their separation unusually hard; the same pairing occurs for Nb–Ta, Mo–W.
• Basicity of Ln(OH)₃ decreases from La(OH)₃ to Lu(OH)₃ because smaller, denser ions polarise O–H bonds more strongly.
• Difficulty in separating lanthanoids from one another — solved industrially by ion-exchange chromatography using a cation-exchange resin and complexing eluents like α-hydroxyisobutyric acid.

Oxidation States

+3 is universal for lanthanoids (La³⁺, Gd³⁺, Lu³⁺ all very stable). Anomalous states appear when an f⁰, f⁷, or f¹⁴ configuration is reached:

• Ce⁴⁺ (f⁰) — strong oxidiser in aqueous solution; CeO₂ is the stable oxide.
• Eu²⁺, Yb²⁺ (f⁷, f¹⁴) — reducing; exist as Sr²⁺-structural analogues in solids.

Actinoids display the full range +3 to +7 (U³⁺ → UO₂²⁺ with U in +6), because 5f orbitals extend further and overlap with ligand orbitals, allowing variable participation in bonding.

Magnetism and Colour

Most Ln³⁺ ions are paramagnetic because of unpaired 4f electrons; the spin-only formula μ = √(n(n+2)) BM works because orbital contribution from f-orbitals is small. For Gd³⁺ (n = 7), μ ≈ 7.94 BM, matching experiment closely. Ln³⁺ ions are mostly colourless because f–f transitions are Laporte-forbidden; visible colour arises only when the energy gap falls in the visible range (e.g., Pr³⁺ green, Nd³⁺ violet, Er³⁺ pink). Actinoid ions are more intensely coloured because 5f orbitals are less shielded and the transitions gain intensity.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Actinoid Contraction and 5f vs 4f Behaviour

The actinoid contraction (Ac³⁺ ≈ 112 pm → Lr³⁺ ≈ 88 pm, ~24 pm) is larger than the lanthanoid contraction for two reasons: (i) the nuclear charge increment is greater, and (ii) 5f electrons shield even less effectively than 4f electrons. Early actinoids (Th–Pu) resemble d-block elements — UO₂²⁺, NpO₂⁺ show covalent, directional bonding — but from Am onward, 5f orbitals contract and behave more like the localised 4f set, matching lanthanoid-like chemistry.

Nuclear and Radiochemical Aspects

Every actinoid is radioactive; only Th, Pa, and U occur in weighable natural abundance (Th-232, U-238, U-235). Elements beyond U (Np, Z = 93, onward) are transuranic, synthesised by neutron capture or heavy-ion bombardment (e.g., Cm by α-bombardment of Pu). Pu-239, bred from U-238, is fissile and central to nuclear fuel cycles.

Reprocessing of spent fuel uses solvent extraction: U and Pu are selectively extracted into tributyl phosphate (TBP) in kerosene from HNO₃ solutions, while fission products remain in the aqueous phase — the PUREX process.

Common Mistakes to Avoid

• Writing Gd as [Xe] 4f⁸ 6s² — the half-filled 4f⁷ 5d¹ 6s² is correct.
• Using μ = √(n(n+2)) for heavy lanthanoids where spin–orbit coupling perturbs the value — the formula is exact only for Gd³⁺.
• Assuming all lanthanoids are coloured; La³⁺, Lu³⁺, Ce⁴⁺ (f⁰), Yb²⁺ (f¹⁴) are colourless because no f–f transition is possible.
• Confusing lanthanoid with lanthanide — the suffix “-ide” traditionally denotes an anion; IUPAC now prefers “-oid”.

Practice Prompts

1. The ionic radius of Zr⁴⁺ is 79 pm; predict the radius of Hf⁴⁺ and justify using lanthanoid contraction. (Expected answer: ~78 pm; nearly identical because of intervening lanthanoid contraction.)
2. Gd³⁺ has the configuration [Xe] 4f⁷. Calculate its spin-only magnetic moment and predict whether its complexes are intensely coloured. (Expected: μ ≈ 7.94 BM; weakly coloured because f–f transitions from a half-filled shell are spin- and Laporte-forbidden.)

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