Lanthanum

Lanthanum
Atomic number	57
Symbol	La
Group	3 (lanthanides)
Boiling point	3460 °C
Electron configuration	[Xe] 5d1 6s2
Density	6.15 g/cm^3
Period	6
Melting point	920 °C
Phase STP	Solid
Block	f
Oxidation states	+3
Wikidata	Q1801

Lanthanum (symbol La, atomic number 57) is a soft, silvery-white metal, the first element of the lanthanide (rare-earth) series in period 6 of the periodic table It is often classed with the rare earth metals, though actually only the 28th most abundant element in Earth’s crust (about 39 mg/kg) Lanthanum normally forms the +3 oxidation state (losing three valence electrons) and is solid (metallic) at standard conditions Its standard atomic weight is about 138.905. In bulk it tarnishes slowly in air and burns to lanthanum oxide (La₂O₃)

• Symbol: La
• Atomic number (Z): 57
• Atomic weight: 138.905 (standard atomic weight)
• Electronic configuration: [Xe] 5d¹ 6s² (three valence electrons)
• Block/series: f-block (lanthanide series); period 6.
• Common oxidation states: +3 (dominant); +2 in a few compounds
• State at STP: Solid metal; soft and ductile (malleable).
• Colour/Appearance: Silvery-white with tarnishing in air.
• Category: Lanthanoid (rare earth metal).

Atomic Structure and Periodic Properties

A lanthanum atom has 57 electrons distributed as [Xe] 5d¹ 6s² In forming compounds, La typically loses all three outer 5d and 6s electrons to become La³⁺, attaining the xenon core configuration Unlike most other lanthanides, neutral lanthanum has no 4f electrons, so it is only weakly paramagnetic in the metal state (However, in compounds the 4f shell can start to become partially occupied.) The triply-charged La³⁺ ion is large and behaves as a pure Lewis acid (no 4f or 5d electrons to color its solutions).

Lanthanum exhibits the expected atomic trends of the lanthanoid series. It has the largest atomic and ionic radius of the lanthanides Typical measures give a nonbonded atomic radius of about 243 pm (van der Waals radius) and a covalent radius around 194 pm Its ionic radius for La³⁺ (coordination 9) is roughly 1.06–1.22 Å. Lanthanum is the most electropositive of the series: its Pauling electronegativity is only ~1.10 The first ionization energy of La (energy to remove one electron) is about 538 kJ/mol which is relatively low and reflects its large size and readiness to give up electrons. (This first ionization energy is lower than that of its lighter alkaline‐earth neighbour barium or calcium, indicating stronger reducing character.) In general, lanthanum is a very strong reducing agent (E° for La³⁺→La is about –2.38 V consistent with its highly reactive metal properties.

Isotopes and Nuclear Properties

Naturally occurring lanthanum consists of two isotopes: ¹³⁹La and ¹³⁸La The only stable isotope is ¹³⁹La (Z=57, N=82), which makes up about 99.91% of natural lanthanum The rare ¹³⁸La (0.09% abundance) is a primordial radioisotope with a very long half-life (~1.05×10^11 years) It decays by roughly equal β⁻ and electron-capture decay to ¹³⁸Ce and ¹³⁸Ba All other lanthanum isotopes are synthetic: the next most important is short-lived ¹⁴⁰La (from neutron activation or fission) with a half-life of about 1.68 days, which shows up as a fission product of uranium Isotopes lighter than ¹³⁸ (like ¹³⁷La, 137k yr half-life) or heavier than ¹⁴⁰ have very short half-lives (seconds or less).

Lanthanum-139 has nuclear spin 7/2⁺ and a magnetic dipole moment of about +2.78 nuclear magnetons making it NMR-active (in fact, ¹³⁹La NMR is used to study La complexes). ¹³⁸La has spin 5⁺ The presence of the long-lived ¹³⁸La has even inspired a radiogeochemical dating method: a “La–Ce isochron” dating technique uses the slow decay of ¹³⁸La to ¹³⁸Ce in rocks, yielding ages comparable to Sm–Nd dates However, in practice lanthanum’s isotopes are not commonly used in routine radioisotopic dating; most isotopes beyond ¹³⁹La are of interest only in nuclear physics or fission product tracking.

Allotropes and Typical Compounds

Allotropes (Metal Phases): Lanthanum metal has three solid phases (allotropes) depending on temperature At room temperature it has the α-La structure, which is hexagonal close-packed. Heating to about 310 °C converts it to β-La (face-centered cubic), and above 865 °C it transforms to γ-La (body-centered cubic) These phases are just different metallic crystal forms; there are no allotropes of lanthanum analogous to carbon’s graphite/diamond or oxygen’s O₂/O₃. All these metal forms are silvery and relatively soft (lanthanum is one of the softest metals, similarly soft or softer than calcium).

Oxides and Hydroxides: Lanthanum oxide (La₂O₃, often called lanthana) is the principal oxide. It is a white solid that forms when lanthanum metal is burned or when soluble La salts are heated. Because La³⁺ is large and polarizing, La₂O₃ adopts a hexagonal “La₂O₃ structure” in the solid state (7-coordinate La) changing to a 6-coordinate structure at high T La₂O₃ is strongly basic: it reacts with water to give lanthanum hydroxide La(OH)₃, releasing heat and even a “hissing” sound Like other rare-earth oxides, La₂O₃ does not form a protective passive layer on the metal, so lanthanum metal slowly corrodes as oxide flakes off. The hydroxide La(OH)₃ itself is only modestly soluble, and in air it further absorbs CO₂ to form basic carbonates. Lanthanum carbonate (e.g. La₂(CO₃)₃) is an important compound (used medically as a phosphate binder in dialysis patients). Lanthanum oxide is widely used industrially in specialty optical glasses, where adding La₂O₃ increases refractive index and improves clarity

Halides and Salts: Lanthanum forms trihalides LaX₃ with all the halogens (X = F, Cl, Br, I). The fluoride LaF₃ is notably insoluble in water (used to test for La³⁺) while the other halides (Cl₃, Br₃, I₃) are deliquescent solids in their hydrated forms. Anhydrous LaCl₃ (and Br₃, I₃) can be made by direct combination of the element with chlorine (or heating the hydrated salt). These LaX₃ salts are colorless ionic crystals with La in +3. Lanthanum also forms oxyhalides like LaOCl when lanthanum chloride is heated (the water of hydration is lost and converts to oxide/chloride)

Hydrides: Upon exposure to hydrogen, lanthanum metal can absorb a large amount, first forming a non-stoichiometric black dihydride LaH₂ (fluorite-type structure, metallic conductor) and then the more salt-like trihydride LaH₃ (which is insulating) These hydrides are pyrophoric in air. The ability of La-based alloys to absorb hydrogen is exploited in nickel–metal hydride (NiMH) battery electrodes.

Other Compounds: Lanthanum forms a variety of compounds typical for large trivalent cations. For example, the sulfate La₂(SO₄)₃, nitrate La(NO₃)₃, phosphate LaPO₄ (a stable solid), and oxide fluorides or borides with extended structures are known. Lanthanum nitride (LaN) and carbide (LaC₂) can be made by direct combination with N₂ or C at high temperature, though these are less common. An important specialized compound is lanthanum hexaboride (LaB₆), a very refractory ceramic (melting ~2200 °C) with a low work function; LaB₆ is used as a hot cathode (electron emitter) in electron microscopes and vacuum tubes. (This compound is extremely stable, purple-black in color, and conducts electricity.)

In summary, lanthanum’s chemistry is dominated by La³⁺ in ionic solids. It has no true molecular allotropes, but its metal and oxide form notable structures. Common bonding motifs are ionic lattices with La³⁺ surrounded by 6–12 anions (F⁻, O²⁻, etc.), reflecting the large ionic radius and coordination preference of La.

Physical Properties

Lanthanum metal is relatively light and soft for a metal. Its density is about 6.15 g/cm³ at room temperature (about 6150 kg/m³). It has one of the lowest melting points of the lanthanoids (about 920 °C) it boils around 3464 °C In pure form it is malleable and easily cut with a knife, as soft as lead or tin, and it is ductile. Freshly made lanthanum metal has luster, but it tarnishes quickly in air (forming lanthanum oxide) and burns readily when ignited

At room temperature lanthanum has a hexagonal close-packed crystal structure (hcp, often called α-La) The electrical resistivity of lanthanum metal at 20 °C is relatively high for a metal, about 615 nΩ·m (compare: aluminum is ~26.5 nΩ·m). Thus its electrical and thermal conductivity are only modest – better than an oxide but far worse than copper or aluminum. Like most lanthanoids, lanthanum is a good paramagnet: however unlike the heavier lanthanides it is only very weakly paramagnetic because the 4f shell is empty in the atom (It has no unpaired 4f electrons.) Lanthanum’s magnetic susceptibility is small and positive.

Optically, lanthanum metal is highly reflective right after polishing, but it quickly loses shine as it oxidizes. In flame tests, lanthanum does not produce a strongly characteristic color distinct from other rare earths. Its compounds, however, find use in optical materials. For example, lanthanum-doped glass is famous; lanthanum oxide and other Lathanum-containing glasses transmit UV light well and improve refractive index for camera lenses Lanthanum compounds also figure in specialized light-emitting devices (e.g. lanthanum halide scintillators and lanthanum-activated phosphors). In stellar spectroscopy, lanthanum’s oxide (LaO) creates distinctive absorption bands in the cool stars, serving as an identification marker

Thermally, the metal has a moderate coefficient of expansion (about 32×10^–6 K^–1 at 25 °C) and a heat capacity around 0.2 J/g·K. In solid state physics experiments, pure lanthanum shows superconductivity below 6–7 K when extremely pure and properly treated, but this property has no practical application beyond research.

Chemical Reactivity and Trends

Lanthanum is a very reactive, electropositive metal – indeed the most reactive of the lanthanides. In air it burns readily: a piece of lanthanum will quickly form La₂O₃ and glow brightly if ignited in a flame. La₂O₃ is strongly basic: it behaves almost like CaO (calcium oxide) On room-temperature air exposure a piece of La metal tarnishes (usually to a dull black oxide/carbide mix) within hours and will corrode completely in a year as oxide flakes off.

Lanthanum reacts easily with halogens. It combines with fluorine, chlorine, bromine or iodine at or near room temperature to produce LaX₃ (X=F,Cl,Br,I) These trihalides are stable ionic solids. Lanthanum metal also reacts with other nonmetals when warmed: for example, lanthanum will reduce nitrogen to form LaN at ~1000 °C, combine with carbon to form LaC₂ (lanthanum carbide) at high T, and react with hydrogen (see above) to form hydrides. With sulfur it can form La₂S₃ at red heat. It catalyzes the decomposition of water, so it slowly liberates hydrogen when in contact with steam or hot water: La + H₂O → La(OH)₃ + 3/2 H₂ (though the reaction is slow at room temp).

In dilute acids lanthanum dissolves vigorously, forming colorless La³⁺ solutions (because La³⁺ has no electrons in d- or f-shell). For example, in sulfuric acid it yields [La(H₂O)₉]³⁺ complexes It does not passivate in mild acid, so it is more active than aluminum or magnesium, dissolving to give La salts. The trivalent ion is strongly Lewis-acidic; lanthanum salts (like La(NO₃)₃ or LaCl₃) hydrolyze easily in water (consistent with La₂O₃ being a strong base).

Lanthanum’s redox chemistry is almost entirely La³⁺↔La^0; higher oxidation states (+4) do not occur and +2 species are very rare and unstable. La^0 is such a strong reductant (E° ~ –2.38 V that it will reduce water, hydrogen ions, and most other metal ions down the list. In reactivity series terms, lanthanum is clearly ahead of hydrogen and aluminum: a piece of Ca or La metal in water will produce H₂ gas. However, La metal does not react with liquid nitrogen or oxygen at –196 °C (unlike alkali metals) – at those low temperatures it is quite unreactive.

Trends: Across the lanthanides from La to Lu, atomic/ionic radii shrink (the “lanthanide contraction”), so later lanthanoids typically form slightly less basic oxides and are marginally less reactive. As the largest lanthanoid, La gives the strongest basic oxide and is chemically the most active of the series It readily forms nine-coordinated complexes (even [La(H₂O)₉]³⁺ in water) and generally binds oxygen and fluorine strongly, reflecting its large ionic radius and +3 charge.

In summary, lanthanum behaves chemically like a highly reactive group-2 (alkaline earth) metal: it is a strong reductant, forms a basic oxide/hydroxide, and dissolves quickly in acids. It is much more reactive (less noble) than the transition metals or most post-transition metals. Its chemistry is dominated by ionic La³⁺ species.

Occurrence and Production

Lanthanum is plentiful on Earth but is not often found alone. In nature, La occurs together with its fellow light rare-earth elements, especially cerium. Major minerals containing lanthanum include monazite (a phosphate, generally Ce- and La-rich) and bastnäsite (a carbonate-fluoride, Ce,La,Nd(Y)CO₃F) In fact, typical bastnäsite ore can be ~38% lanthanum by rare-earth content, and monazite ~25% Lanthanum is usually not the dominant rare-earth in an ore (cerium usually is), although there are rare minerals like monazite-(La) or lanthanite-(La) where La leads Overall, lanthanum averages about 39 mg/kg in Earth’s crust making it about three times as common as lead (Pb) and more abundant than many “common” transition metals.

Modern production of lanthanum comes largely from processing rare-earth ores. The ore is mined (often as a mixed rare-earth concentrate) and first treated to remove thorium (a radioactive impurity) and other gangue. For example, a typical process for bastnäsite involves crushing the ore, then digesting it with concentrated H₂SO₄, filtering, and leaching; this yields a solution rich in rare-earth sulfates, from which the rare earths (including La) are precipitated as hydroxides or oxalates. Monazite (which has thorium) requires additional steps to precipitate thorium and selectively convert Ce to insoluble CeO₂

After initial separation, lanthanum is typically separated from its lanthanide siblings by fractionation methods. One historical method is fractional crystallization of double salts (e.g. ammonium lanthanum nitrate double salts) Modern plants usually use liquid–liquid (solvent) extraction or ion-exchange chromatography exploiting the slight solubility differences among LREE (light rare earths) salts. For instance, Ce³⁺ can be oxidized to Ce⁴⁺ (forming CeO₂), which precipitates out, leaving La³⁺ in solution for further purification Once lanthanum has been isolated as a pure oxide or salt, it can be converted to metal.

Lanthanum metal is usually made by reducing lanthanum compounds. A common route is metallothermic reduction: La₂O₃ or LaCl₃ is reduced with calcium metal or other strong reducers at 800–1000 °C. For example, LaCl₃ is melted with Ca, producing La metal and CaCl₂ Another method is to reduce LaF₃ with calcium or to co-reduce La₂O₃ with Na/Li alloys (thermite-like reactions). Large-scale pure lanthanum metal production is limited; most commercial use is in the form of La₂O₃ or alloys rather than as the pure metal.

Major current producers of lanthanum (as part of rare-earth production) include China (by far the largest globally), as well as the United States (e.g. Mountain Pass, California), Myanmar, Australia and India. (China provides roughly 70% or more of world rare-earth output Many countries are exploring or restarting rare-earth production to diversify supply, since lanthanum is essential for high-tech uses. European deposits (e.g. in Sweden and Norway) and North American sites are also under development, but for now China, with its mines in Inner Mongolia and Sichuan, dominates.

Applications and Technology

Lanthanum and its compounds have diverse high-technology applications, often capitalizing on its unique electronic and physical properties. Key uses include:

• Hydrogen storage and batteries: Lanthanum–nickel alloys (such as LaNi₅) are famous for their ability to absorb and release hydrogen. These metal hydrides store hydrogen for fuel-cell and NiMH battery applications. In fact, sintered alloys of lanthanum with nickel and other light rare earths compose the negative electrode (anode) in nickel–metal hydride batteries used in hybrid vehicles. In such batteries, LaNi₅-type alloys can reversibly absorb ~6 hydrogen atoms per formula unit. La’s large atomic volume and valence ease hydrogen uptake, making these alloys highly effective and rechargeable.

• Mischmetal/flints: Lanthanum is a principal component of mischmetal, an alloy of several rare earths (about 20% La) used to make lighter flints. When scraped, the alloy sparks by igniting lanthana (La₂O₃) mixed with iron. Such flints (so-called ferrocerium or “lighter flints”) have replaced natural flints. The presence of La and other REEs (like Ce, Nd) ensures easy ignition and glowing sparks, which are ideal for igniting gas valves in cigarette lighters and welding torches.

• Optical glass: Lanthanum oxide (and other La compounds) significantly improve optical glass. Adding up to ~10–20% La₂O₃ to silicate glass raises its refractive index (n) and lowers dispersion, resulting in high-quality crown and flint glasses. This glass is used in precision camera lenses, binoculars, and wide-aperture eyepieces. Lanthanum-containing glass also has high UV transparency and chemical durability. Older “lanthanum glass” formulations greatly enhanced photographic and projection optics Rare-earth optical glasses, including lanthanum, helped modernize cinematography projectors and studio lighting (objective: a light similar to daylight).

• Catalysts and refining: Lanthanum compounds are widely used in catalysts, especially for petroleum refining. For example, lanthanum oxide and ceria form the base of so-called “REE catalysts” for fluid catalytic cracking (FCC) in oil refineries; lanthanum’s role is to stabilize the alumina support and improve activity. The RSC notes that lanthanum salts serve in petroleum catalysts In organic chemistry, lanthanum trichloride (LaCl₃) or lanthanum triflate (La(OTf)₃) are Lewis-acid catalysts for certain rearrangements or coordination processes. Lanthanum-doped catalysts are also explored for biodiesel production and in automotive emission control.

• Lighting and electrons: Compounds of lanthanum emit bright light under excitation. For instance, carbon arc lamps and discharge lamps often contain lanthanum additives to produce a bright white light with a spectrum close to sunlight. Lanthanum oxide electrodes are used in high-intensity carbon arcs (e.g. for cinema projectors and studio lights). Lanthanum also improves electron-emission cathodes: lanthanum hexaboride (LaB₆) is a premier electron emitter, used as cathodes in electron microscopes, X-ray tubes, and CRTs due to its low work function and long life. (LaB₆ cathodes enable brighter beams at lower temperatures than tungsten cathodes.)

• NiMH and NiCd batteries: Beyond hydrogen storage, lanthanum is also used in NiCd (nickel–cadmium) battery alloys to a small extent. More importantly, the NiMH batteries in hybrids use electrodes made from mischmetal (La+Ce-based) hydride alloys as above. Lanthanum’s role in these batteries was crucial to their mid-1990s commercialization, providing high capacity and stability.

• Electronics and ceramics: Lanthanum oxide (La₂O₃) is a high-dielectric-constant (“high-κ”) material and has been studied for use in advanced electronics (gate oxides for transistors). Lanthanum aluminate (LaAlO₃) is a perovskite used in microwave devices and as a substrate for high-T_c superconductors. Lanthanum manganite (LaMnO₃) is a perovskite used in solid oxide fuel cells and as a colossal magnetoresistance material. In glass-ceramic cooktops and insulating ceramics, lanthanum can improve strength and thermal stability.

• Chemical additives and tracer: The La³⁺ ion mimics Ca²⁺ in some biochemical contexts (same charge, similar radius), so radioactive lanthanum isotopes have been used as tracers for calcium-binding in biology. Lanthanum chloride (LaCl₃) is known to be a potent blocker of calcium channels in cells, illustrating this similarity. Lanthanum salts (e.g. LaCl₃) also find occasional use in water purification or as specialized reagents in inorganic synthesis.

• Medicine: One modern application is lanthanum carbonate as a phosphate binder (drug name Fosrenol) in renal failure. When taken orally, insoluble lanthanum carbonate binds dietary phosphate in the gut (LaPO₄ is formed) and prevents phosphate absorption. Over long-term use, patients accumulate lanthanum in bone, but clinical studies report that lanthanum therapy is generally safe, with few severe side effects This pharmaceutical use exploits the low solubility and strong affinity of La³⁺ for phosphate.

In summary, lanthanum’s technological uses capitalize on its involvement in alloys (hydrides, mischmetal), its optical impact (glasses and lighting), and its chemical reactivity (catalysts, ceramics). Demand for lanthanum largely follows the need for rare-earth enabling materials: batteries, glass optics, catalysts, and electronics all rely on it as a component.

Biology, Environment, and Safety

Lanthanum has no known biological role in humans or higher animals. It is considered nonessential. In fact, lanthanum is not considered an essential nutrient and is only sparingly absorbed if ingested However, some soil and bacterial species do appear to utilize lanthanides: certain methylotrophic bacteria require lanthanides (La to Lu) as cofactors for methanol dehydrogenase enzymes. In this very specialized sense, lanthanum can be “essential” to some microbes, but such roles were only discovered in the last decade and have no medical implications.

In general human physiology, lanthanum is poorly absorbed from the gut. Because La³⁺ binds strongly to phosphates, it tends to accumulate in bone and liver when ingested. Clinical experience with lanthanum carbonate (dialysis patients) indicates low toxicity: kidney patients often tolerate the drug well, and serum La levels remain low. No significant carcinogenic or teratogenic effects of lanthanum compounds have been identified in humans. At high exposures, rare-earths can cause bone or soft-tissue accumulation, but lanthanum is not known to cause critical poisoning. Some antibacterial activity of La³⁺ is reported (it can inhibit microbial growth at high concentration)

For the general population and environment, lanthanum is not very mobile or soluble. In soil or water, La³⁺ rapidly binds to carbonate, phosphate, and hydroxide to form insoluble compounds. Therefore, lanthanum introduced into the environment (e.g. by mining or waste) tends to precipitate out quickly and does not bioaccumulate up the food chain. However, any soluble La compounds (like nitrates) should be considered very stable and hard to eliminate. In mining regions, lanthanum and its decay daughter ¹³⁸La (a weak β-emitter) are present in residues, but their radiological hazard is extremely low (¹³⁸La’s half-life is ~10^11 years).

Safety precautions: As a metal, lanthanum is reactive. Finely divided La powder or filings will ignite spontaneously in air and must be handled under inert atmosphere. Solid lanthanum metal should be kept away from acids and water to prevent vigorous reactions. Lanthanum oxide and hydroxide are alkaline; contact can cause irritation to skin or eyes. Inhalation of lanthanum dust is the chief occupational hazard – like other metal dusts, it can cause respiratory irritation and, with chronic exposure, a rare-earth pneumoconiosis. Nevertheless, lanthanum is less toxic than heavy metals like lead; its LD₅₀ (oral) in animals is quite high (on the order of grams per kilogram body weight).

In laboratory and industrial practice, lanthanum compounds are handled with standard precautions: gloves, eye protection, and fume hoods as needed. Owing to its slow biological uptake, lanthanum salts are generally assigned low hazard ratings (e.g. moderately toxic). Potentiometric detectors often ignore La, treating it as inert. There are no strict regulatory exposure limits for lanthanum specifically, but it is treated like other non-radioactive rare-earths: avoid dust inhalation and minimize skin contact. Surface water or drinking water standards do not typically include lanthanum, reflecting its negligible solubility.

In summary, lanthanum poses minimal environmental or health risk under normal conditions. It has no nutrient role in humans, exhibits low chemical toxicity, and any released La generally precipitates and remains in soil. Its most important hazards are those common to reactive metals and metal powders.

History and Etymology

Lanthanum’s history began in the early 19th century. In 1751 the Swedish chemist Axel Cronstedt discovered the mineral cerite (initially as an unidentified substance), which later was found to contain cerium. Later, in 1803, Berzelius and Hisinger isolated ceria (CeO₂) from a Bastnäs mine sample Not until 1839 did Carl Gustaf Mosander (another Swede, a student of Berzelius) carefully analyze ceria. He discovered that “ceria” was actually a mixture of oxides: one portion was called lanthana (lanthanum oxide) and another didymia (Mosander separated cerium into “ceria” and “lanthana,” the latter containing our lanthanum.) Independently that same year, another chemist Axel Erdmann found lanthana in a Norwegian mineral. Because the new element-code number could not be seen in the older cerium sample, Mosander named it lanthana from the Greek lanthanein (“to lie hidden”) The name “lanthanum” (Latinized form) literally means “the hidden one.”

For many years following 1839, lanthanum was recognized only in oxide form or mixed with cerium. It was not isolated as pure metal until the 1920s. The first true metallic lanthanum was prepared in 1923 by W. H. Kroll, who reduced lanthanum chloride with calcium (independently of the more famous Kroll process for titanium). Early chemists had intermittent confusion (for example, Mosander later thought lanthana also contained a second element didymium, which itself turned out to be a mix of praseodymium and neodymium). But by the 1940s the lanthanide series was well-defined and lanthanum firmly established as the first member (Z=57).

Lanthanum was long called a “rare earth” reflecting the old terminology for several oxides that were hard to separate by 19th-century chemistry The term is historical; as noted, La is moderately abundant. It was also called “lanthanon” in some early texts (from Greek lanthano, hidden). Its element symbol La comes from the name lanthanum. There are no alternative names in modern usage (unlike its fellow 19th-century “mystery mix” didymium).

Industrial utilization of lanthanum expanded in the mid-20th century. Its role in lighter flints (mischmetal, ~1900s) and high-index glass (around WWII) were early applications. Nuclear research in the 1940s and 50s characterized its isotopes (¹³⁸La, ¹³⁹La, etc.) and reactivity. The advent of NiMH batteries in the 1980s–90s greatly boosted lanthanum demand for LaNi₅ alloys. Photographic and optical technologies (like Kodak’s lanthanum glass) and petroleum catalysts (beginning 1970s) also drove usage. More recently, medical use (lanthanum carbonate) and electronic research have kept lanthanum important.

In sum, lanthanum’s story is that of “the hidden one” finally finding its place: discovered in 1839 within cerium, named for its concealment, isolated as metal in 1923, and today recognized as a crucial element for modern catalysts, optics, batteries, and other advanced materials.

Data Table

Property	Value
Symbol / Name	La / Lanthanum
Atomic number (Z)	57
Standard atomic weight	138.905 (unified atomic mass units)
Period / Block / Category	6 / f-block / Lanthanoid (rare earth)
Electron configuration (ground)	[Xe] 5d¹ 6s²
Common oxidation states	+3 (predominant); +2 (rare)
Atomic radius (covalent)	~194 pm (Å)
Atomic radius (nonbonded)	~243 pm (Å)
Electronegativity (Pauling)	1.10
First ionization energy	538 kJ/mol (≈5.57 eV)
Density (20 °C)	6.15 g/cm³ (≈6150 kg/m³)
Melting point	920 °C (1193 K)
Boiling point	3464 °C (3737 K)
Crystal structure (RT)	Hexagonal close-packed (α-La)
Key stable isotope	¹³⁹La (99.9% nat’l abundance)
^138La half-life	~1.05×10^11 years (β⁻ + electron-capture decay)
CAS Registry Number	7439-91-0
Discovery	1839, Carl G. Mosander (oxide)
Etymology	Greek lanthanein "to lie hidden" (lanthanum = "the hidden one")

Sources: Standard reference data for lanthanum are compiled from chemistry handbooks and recent literature