Lanthanoids & Actinoids (Inner-Transition Elements) Meet the inner‑transition elements (f‑block) Where the f‑block sits: lanthanoids (4f series) and actinoids (5f series) shown as two rows detached below the main periodic table. Insets show the steady radius decrease called lanthanoid/actinoid contraction. Inner‑transition elements are the f‑block. Lanthanoids are the 14 elements following lanthanum, from cerium (Ce, Z=58) to lutetium (Lu, Z=71). Actinoids are the 14 elements following actinium, from thorium (Th, Z=90) to lawrencium (Lr, Z=103). They fill 4f (lanthanoids) and 5f (actinoids) orbitals. Their chemistry features a dominant +3 state for lanthanoids, more variable states (up to +7) for actinoids, weak but characteristic f–f colors, and strong paramagnetism. Filling of 4f across Ce to Lu; occasional 5d occupancy (0–1). Lanthanide general configuration Actinide general configuration Filling of 5f across Th to Lr; occasional 6d occupancy (0–1). Quick facts (NEET‑level) Lanthanoid series: 14 elements from Ce (58) to Lu (71) — after La. Actinoid series: 14 elements from Th (90) to Lr (103) — after Ac. +3 is the most stable oxidation state for lanthanoids; actinoids show +3 to +6 commonly, and even +7 in some early members (e.g., Np, Pu). Lanthanoid contraction: metallic/ionic radii decrease smoothly from La to Lu due to poor f‑electron shielding. Most lanthanoids are not radioactive (promethium, Pm, is an exception). Most actinoids are radioactive. Electronic structure and oxidation states Why this matters: Configuration drives oxidation state and reactivity. In lanthanoids, the 4f levels are buried and do not participate much in bonding. This makes +3 (loss of two 6s and one 5d/4f electron) the most stable. Two famous exceptions: Ce 4+ is unusually stable because it attains the noble‑gas‑like [Xe] 4f 0 state and acts as a strong oxidizing agent; Eu 2+ is relatively stable due to its half‑filled 4f 7 configuration (extra stability). Some others like Yb 2+ (4f 14 ) and Sm 2+ can appear in suitable conditions, but +3 dominates. Actinoids have 5f electrons that are more extended and interact with the environment; losing them is easier. Hence a wide range of oxidation states (+3, +4, +5, +6, and occasionally +7) is observed, especially for early actinoids. Ion Electronic reason Remark Selected stable/accessible lanthanoid oxidation states (beyond +3) Lanthanoid Ce 4+ 4f 0 (noble‑gas‑like) Strong oxidizing agent (Ce(IV) salts like ceric ammonium nitrate). Eu 2+ 4f 7 (half‑filled) Relatively stable; gives pale colors; useful in phosphors. Yb 2+ 4f 14 (filled) Observed in some solids/solutions; powerful reductant. Tb 4+ Near half‑filled stability in solids Exists in TbO2; less common in aqueous chemistry. neet-alert High‑yield: Ce 4+ is stabilized by 4f 0 and behaves as a strong oxidizing agent; Eu 2+ is stabilized by 4f 7 (half‑filled). +3 remains the dominant state for lanthanoids. Lanthanoid contraction — small change, big consequences Definition: Lanthanoid contraction is the steady decrease in atomic/ionic size from La to Lu. Cause: 4f electrons shield each other poorly, so effective nuclear charge felt by outer electrons increases across the series. Observation: The metallic (atomic) radius decreases from about La ≈ 1.72 Å to Lu ≈ 1.56 Å (representative values). The same trend is seen for Ln 3+ ionic radii. Why it matters: This subtle shrinkage has major knock‑on effects across the periodic table, especially making 4d and 5d transition‑metal pairs almost the same size (e.g., Zr≈Hf), complicating their separation and aligning many of their properties. 2026-05-26T17:05:05.751Z Line graph of lanthanoid contraction: x-axis Z (57 to 71), y-axis metallic radius (Å). Smooth decreasing curve from ~1.72 Å (La) to ~1.56 Å (Lu). Include faint markers for La3+ to Lu3+ ionic radii trend. Clean 2D vector style, red curve, axes labeled, white background. Graph of lanthanoid contraction: plot of atomic/ionic radius vs atomic number from La to Lu showing a smooth downward slope. gpt-image-2 Zr–Hf, Nb–Ta, Mo–W radii similarity 5d elements are pulled in by prior 4f contraction → sizes close to 4d analogues Zr≈Hf; Nb≈Ta; Mo≈W — chemical separation hard; properties very similar Basicity of Ln(OH)3 decreases La→Lu Smaller Ln 3+ is more polarizing → O–H bonds less free to donate OH− La(OH)3 > ... > Lu(OH)3 (basicity trend) Separation of lanthanoids is tedious Similar sizes/chemistry across series Requires ion‑exchange or solvent extraction Effect Consequence Explanation NEET angle Consequences of lanthanoid contraction 2026-05-26T17:05:05.868Z Side-by-side bar chart comparing metallic radii: Zr vs Hf, Nb vs Ta, Mo vs W. Show near-equal heights in each pair. Clear labels, neutral palette, vector style, no internal text beyond element symbols and radii scale. Size twins: Zr vs Hf, Nb vs Ta, Mo vs W — bars show nearly equal metallic radii for 4d and 5d pairs due to lanthanoid contraction. gpt-image-2 Colors and magnetism of lanthanoid ions Colors: Most Ln 3+ ions are colorless to faintly colored because f–f transitions are Laporte‑forbidden and the 4f orbitals are well shielded (sharp spectral lines, weak intensity). This is unlike d–d transitions in transition metals, which are stronger. Magnetism: Lanthanoid ions are strongly paramagnetic due to unpaired 4f electrons. For Ln 3+ , spin‑only formulas do not work; total angular momentum (J) with Russell–Saunders (L–S) coupling is needed. Magnetic moment (lanthanoids, L–S coupling) g is the Landé g‑factor; J is total angular momentum. This differs from the spin‑only formula used for many 3d ions. Ion (aq/solid context) Color/emission (typical) Key use Selected lanthanoid ions: colors and uses Ce 3+ Colorless to very pale (shielded f–f) Glass decolorizer; polishing powders (CeO2) Nd 3+ Lilac/pink tints; sharp lines Nd:YAG lasers; neodymium glass Eu 3+ Red emission in phosphors Y2O3:Eu — red phosphor in CRT/CFL/LED Tb 3+ Green emission La2O2S:Tb — green phosphor Dy 3+ Yellow‑white emission (phosphors) Lighting phosphor blends Separation of lanthanoids Because lanthanoids have very similar ionic sizes and chemistry, they are difficult to separate by classical precipitation. Industrial routes use: - Ion‑exchange chromatography: Ln 3+ ions are loaded on a cation‑exchange resin and eluted (often with complexing agents like citrate/EDTA) to exploit small differences in complex stability. - Solvent extraction: an organic extractant (e.g., organophosphorus acids) selectively transfers certain Ln 3+ to the organic phase. Repeated stages achieve separation. 2026-05-26T17:05:06.013Z Ion‑exchange separation concept: Ln3+ mixture binds to resin, then different lanthanoids elute at slightly different times with complexing eluent. Schematic of cation-exchange column: packed resin beads, feed of mixed Ln3+ at top, eluent with complexing agent, separated colored bands eluting. Labels: 'Resin', 'Feed', 'Eluent', 'Fractions'. Clean vector diagram. gpt-image-2 Uses of lanthanoids — from sparks to super‑magnets Misch metal: An alloy typically ≈ Ce 50% + La 25% + Nd 18% + others, used in lighter flints — it sparks when scraped due to rapid oxidation. NdFeB magnets: Neodymium–iron–boron magnets are among the strongest permanent magnets, powering headphones, phone speakers, brushless motors, EVs, and wind‑turbine generators. Phosphors: Eu 3+ in Y2O3:Eu gives red; Tb 3+ in La2O2S:Tb gives green — used in CRTs, CFLs, and LEDs. Lasers: Nd:YAG (neodymium‑doped yttrium aluminum garnet) emits at 1064 nm, used for cutting, surgery, and ranging. Medicine: Gd 3+ chelates are MRI contrast agents. 2026-05-26T17:05:07.097Z Illustration showing a compact NdFeB rotor in a brushless DC motor and a wind turbine nacelle cutaway highlighting NdFeB magnet generator. Labels: 'NdFeB magnet', 'Rotor', 'Generator'. Clean engineering vector style. NdFeB magnets enabling EV motors and wind turbines — strongest permanent magnets in daily tech. gpt-image-2 remember Tech in your pocket: Neodymium (Nd) is in tiny but powerful speakers; Gd‑chelates brighten MRI scans; Eu and Tb make display colors pop. Actinoids overview and oxidation states Lanthanoids vs actinoids at a glance: filling (4f vs 5f), oxidation states (+3 dominant vs wide range), radioactivity, and complex-formation tendencies. Actinoids (Th to Lr) fill 5f orbitals. Most are radioactive; only Th, U, and Pa occur significantly in nature. Because 5f electrons extend further and interact more with the environment than 4f, actinoids show more covalency and many oxidation states: +3 and +4 are common; +5 and +6 appear readily (e.g., UO2 2+ ); +7 occurs for some early actinoids (e.g., neptunium, plutonium) in strong oxidizing conditions. Thorium: Th 4+ is especially stable (no 5f in Th 4+ ); important in Th‑based nuclear cycles. Uranium: U 3+ , U 4+ , U 5+ , U 6+ are known; UO2 2+ (uranyl) is a key +6 oxo‑cation (uranyl salts are typically yellow and may show greenish fluorescence in glass). Neptunium/Plutonium: access high states in oxo complexes (up to +7 under suitable conditions). Examples (oxidation‑state diversity) Thorium and uranium in nuclear technology Thorium: India has abundant Th in monazite sands; Th 4+ is chemically stable. In breeder reactors, 232 Th can be converted to fissile 233 U. Uranium: Natural uranium contains 235 U (fissile) and 238 U. Uranium is converted to UF6 (uranium(VI) fluoride) for isotope separation by gaseous diffusion or centrifugation. In water reactors, 235 U undergoes fission, releasing energy and neutrons. 238 U can breed 239 Pu via neutron capture and successive beta decays. Two successive - decays convert 238 U (after neutron capture) to 239 Pu. Breeding Pu‑239 from U‑238 Thorium‑232 breeds fissile uranium‑233 via neutron capture and - decays. Thorium breeder cycle to U‑233 Fission of U‑235 (schematic) Exact fission fragments vary; multiple neutrons sustain the chain reaction and release energy. 2026-05-26T17:05:07.820Z gpt-image-2 Cutaway schematic of a heavy-water or advanced heavy-water reactor adapted for Th cycle: fuel core, Th-232 blanket, moderator, control rods, steam generator, turbine. Labels for each. Clean educational vector diagram. Thorium‑based reactor concept (India): blanket with Th‑232, core with fissile fuel, heat exchangers, and control systems. Lanthanoids vs actinoids — compare and contrast Origin of series 4f filling after La 5f filling after Ac Dominant oxidation state +3 (with some +2/+4 exceptions) Variable: +3 to +6 common; up to +7 in early members Radioactivity Mostly non‑radioactive (Pm is radioactive) Mostly radioactive; only Th, U, Pa occur significantly in nature Color origin Weak f–f transitions (Laporte‑forbidden) f–f and charge‑transfer; often stronger colors Complex formation Hard cations ( Ln 3+ ); mainly ionic complexes Greater covalency; stronger complexing than lanthanoids Separation difficulty High (similar sizes) — ion‑exchange/solvent extraction Challenging and radiologically constrained Industrial highlights Misch metal, NdFeB magnets, phosphors, lasers Nuclear fuel (U, Pu), Th breeder cycles, Am in smoke detectors Feature Lanthanoids (Ce–Lu) Actinoids (Th–Lr) Comparison: Lanthanoids vs Actinoids From magnets to medicine to megawatts: lanthanoids power LEDs, MRI, and motors; actinoids drive nuclear power, detectors, and therapies. neet-alert NEET traps: (1) Lanthanoid contraction explains Zr≈Hf similarity. (2) +3 is most common for lanthanoids but not exclusive ( Ce 4+ , Eu 2+ ). (3) Actinoids show wider oxidation‑state variety than lanthanoids. Lanthanoid contraction affects only the lanthanoid series; it has no impact on other elements. False. The contraction pulls down 5d sizes, making 4d/5d pairs (Zr–Hf, Nb–Ta, Mo–W) almost the same size — a major reason they are hard to separate and have similar properties. All f‑block elements show only the +3 oxidation state. No. +3 dominates for lanthanoids, but +2 (e.g., Eu 2+ ) and +4 (e.g., Ce 4+ ) also occur. Actinoids frequently access +4, +5, +6 (and even +7) due to accessible 5f, 6d, 7s electrons. Most lanthanoids are stable (except promethium). Most actinoids are radioactive; only Th, U, and Pa occur naturally in significant amounts. All lanthanoids and actinoids are radioactive. Key terms f‑block (inner‑transition elements) Elements filling 4f (lanthanoids) or 5f (actinoids) subshells; placed as two rows below the main periodic table. inner transition elements Lanthanoid (lanthanide) The 14 elements Ce to Lu (Z=58–71) following La; characterized by 4f filling. The 14 elements Th to Lr (Z=90–103) following Ac; characterized by 5f filling. Actinoid (actinide) The steady decrease in size from La to Lu due to poor shielding by 4f electrons, increasing effective nuclear charge. Lanthanoid contraction Misch metal A Ce‑rich lanthanoid alloy (≈Ce 50%, La 25%, Nd 18%, others) used in lighter flints. Neodymium–iron–boron permanent magnet material; extremely high magnetic energy product. NdFeB Elements with Z>92 (beyond uranium); all are synthetic (e.g., Np, Pu, Am, Cm). Transuranic elements Half‑filled 4f 7 stability Extra stability for Eu 2+ (and Gd 3+ ) due to symmetrical, half‑filled 4f subshell. Breeder reactor A reactor that converts fertile isotopes (e.g., 232 Th, 238 U) into fissile ones ( 233 U, 239 Pu) using excess neutrons. Monazite sand Phosphate mineral sands rich in Th and rare earths; found on India’s coasts. Coupling of orbital (L) and spin (S) angular momenta giving total J; basis for lanthanoid magnetic‑moment calculations. Russell–Saunders (L–S) coupling