Lanthanides & Actinides: JEE Main Complete Guide

Lanthanides and Actinides (the f-block elements) are among the most commonly underestimated topics in JEE Main Chemistry. Because the chapter is short, students give it one cursory reading and move on — then lose 2–3 marks to questions they could have answered correctly with one focused hour of study. The chapter has only 10–12 core facts that JEE tests; this guide extracts them all and explains why each fact is true, so you remember rather than memorise.

Electronic Configurations: The Core Pattern

Lanthanides fill the 4f subshell: La (Z=57) through Lu (Z=71), with general configuration [Xe] 4f^(1–14) 5d^(0–1) 6s². Actinides fill the 5f subshell: Ac (Z=89) through Lr (Z=103), general configuration [Rn] 5f^(1–14) 6d^(0–1) 7s². JEE asks for specific configurations — remember the exceptions: La [Xe] 5d¹ 6s² (no 4f), Gd [Xe] 4f⁷ 5d¹ 6s² (half-filled 4f), Lu [Xe] 4f¹⁴ 5d¹ 6s². For Actinides: Ac [Rn] 6d¹ 7s² (no 5f), Th [Rn] 6d² 7s², Pa [Rn] 5f² 6d¹ 7s². These exceptions are the ones that appear in JEE.

Oxidation states: Lanthanides predominantly show +3 state (most stable). Ce also shows +4 (oxidising agent — Ce⁴⁺ is used in the lab). Eu and Sm show +2. Actinides show more variable oxidation states (due to comparable 5f, 6d, 7s energies): U shows +3, +4, +5, +6; the +6 state (UO₂²⁺, uranyl ion) is particularly stable. This greater variability is a JEE-tested contrast between lanthanides (mostly +3) and actinides (variable). Take a free f-block elements mock to test your recall speed on these configurations.

Lanthanide Contraction: The Most Important Consequence

Lanthanide contraction is the steady decrease in atomic and ionic radius from La to Lu despite increasing atomic number. Cause: poor shielding by 4f electrons (diffuse orbital, high penetration of outer electrons needed). The 4f electrons do not effectively shield each other from nuclear charge; as Z increases, each new 4f electron experiences nearly the full additional proton charge, pulling the outer electrons inward. Result: ionic radii of La³⁺ (117 pm) to Lu³⁺ (100 pm) decrease by ≈17 pm across 14 elements.

Consequences of lanthanide contraction — this is where JEE focuses: (1) The 5d transition metals of the 2nd and 3rd rows (Zr/Hf, Nb/Ta, Mo/W) have nearly identical atomic sizes because the lanthanide contraction offsets the expected size increase with n. This explains why Hf is unexpectedly dense and chemically similar to Zr. (2) Basicity of lanthanide hydroxides decreases from La(OH)₃ to Lu(OH)₃ as the smaller ion binds OH⁻ more strongly. (3) Separation of lanthanides from each other is difficult because their similar sizes make ion-exchange separations require many steps. For context on d-block contraction and periodicity, see our D and F block elements guide and our periodic properties guide.

Magnetic Properties and Colour

Lanthanide ions are coloured because of f–f electronic transitions — transitions within the 4f subshell. Unlike d-d transitions in transition metals (which are affected by ligand field splitting), 4f transitions are well-shielded from ligand effects and produce sharp absorption lines rather than broad bands. This is why lanthanide lasers (Nd:YAG) produce coherent light at specific wavelengths. JEE tests this as: "Why do lanthanide compounds show sharp spectral lines compared to transition metal complexes?" Answer: 4f orbitals are shielded from the ligand field.

Magnetic moments of lanthanides are calculated using J (total angular momentum) rather than S (spin only), because spin-orbit coupling is large in f-block. μ = g√(J(J+1)) Bohr magnetons, where g is the Landé g-factor. This contrasts with d-block: μ = √(n(n+2)) (spin-only formula works for most 3d metals). JEE sometimes asks why Eu²⁺ and Gd³⁺ are isoelectronic (both have 4f⁷) but Gd³⁺ is more paramagnetic — because Gd³⁺ has all unpaired f electrons with no quenching of orbital contribution. For related coordination chemistry, see our coordination chemistry guide.

Key Actinide Facts for JEE

Actinides vs lanthanides — four contrasts JEE tests: (1) Actinides have greater radioactivity; (2) Actinides show wider range of oxidation states; (3) 5f orbitals are less effectively shielded than 4f (so actinide chemistry shows more covalent character); (4) Actinides form more stable oxo-cations (UO₂²⁺, NpO₂⁺, PuO₂²⁺). Nuclear applications: U-235 and Pu-239 are fissile materials. Th-232 is a fertile material (converts to U-233 by neutron capture). These application facts appear as one-line JEE questions about nuclear reactors or radioactive series.