Lanthanum

Lanthanum, atomic number 57 and symbol La, is a silvery-white metal that is soft and ductile. It tarnishes slowly in air. As the first element in the lanthanide series, it serves as the prototype for that group of 15 similar elements, and it is traditionally classified as a rare-earth element. Its most common oxidation state is +3, though some +2 compounds exist. Lanthanum has no biological role in humans, but certain bacteria use it; it is not very toxic to people but does display some antimicrobial activity.

This element typically appears alongside cerium and other rare-earth elements. Swedish chemist Carl Gustaf Mosander first identified lanthanum in 1839 as an impurity in cerium nitrate, naming it from the ancient Greek *lanthanein*, meaning "to lie hidden." Despite its rare-earth classification, lanthanum is the 28th most abundant element in Earth’s crust—nearly three times as abundant as lead. In minerals like monazite and bastnäsite, it makes up about a quarter of the lanthanide content. Extracting it from these minerals involves such a complex process that pure lanthanum metal was not isolated until 1923.

Lanthanum compounds serve many purposes: as catalysts, glass additives, carbon arc lamps for studio lights and projectors, ignition elements in lighters and torches, electron cathodes, scintillators, and gas tungsten arc welding electrodes. Lanthanum carbonate is used as a phosphate binder to treat high blood phosphate levels linked to kidney failure.

**Physical** Lanthanum sits to the right of barium and left of cerium on the periodic table, and it is generally considered the first f-block element. Its 57 electrons are arranged as [Xe]5d¹6s², with three valence electrons outside the noble gas core. In chemical reactions, it almost always gives up these three electrons to form the +3 oxidation state, reaching the stable xenon configuration. Some +2 compounds exist but are usually much less stable. Lanthanum monoxide (LaO) produces strong absorption bands in certain stellar spectra.

Among lanthanides, lanthanum is unusual because its single gas-phase atom has no 4f electrons. (Similarly, actinium and thorium lack 5f electrons as gas-phase atoms.) As a result, it is only weakly paramagnetic, unlike the strongly paramagnetic later lanthanides (except ytterbium and lutetium, whose 4f shells are full). However, in chemical environments, lanthanum’s 4f shell can become partially occupied and participate in bonding. For instance, the melting points of trivalent lanthanides (excluding europium and ytterbium) relate to how much the 6s, 5d, and 4f electrons hybridize—more 4f involvement lowers the melting point. Lanthanum has the second-lowest melting point among them at 920 °C. (Europium and ytterbium melt even lower because they delocalize about two electrons per atom instead of three.) This chemical availability of f orbitals justifies placing lanthanum in the f-block, despite its anomalous ground-state configuration (which arises from strong interelectronic repulsion making it less favorable to occupy the small, core-close 4f shell).

As the lanthanide series progresses, the metals become harder; lanthanum, as expected, is soft. It has a high electrical resistivity of 615 nΩm at room temperature, compared to aluminum’s 26.50 nΩm. Lanthanum is the least volatile of the lanthanides. At room temperature, it has a hexagonal crystal structure (α-La). At 310 °C, it shifts to face-centered cubic (β-La), and at 865 °C, to body-centered cubic (γ-La).

**Chemical** Following periodic trends, lanthanum has the largest atomic radius among the lanthanides, making it the most reactive. It tarnishes quite rapidly in air, turning completely dark after several hours, and can readily burn to form lanthanum(III) oxide (La₂O₃), which is almost as basic as calcium oxide. A centimeter-sized sample corrodes completely in a year because its oxide spalls off like iron rust, rather than forming a protective coating like aluminum, scandium, yttrium, or lutetium. At room temperature, lanthanum reacts with halogens to form trihalides, and upon warming, it forms binary compounds with nitrogen, carbon, sulfur, phosphorus, boron, selenium, silicon, and arsenic. It reacts slowly with water to produce lanthanum(III) hydroxide (La(OH)₃). In dilute sulfuric acid, it readily forms the colorless aquated tripositive ion [La(H₂O)₉]³⁺, which has no d or f electrons. Lanthanum is the strongest and hardest base among the rare earth elements, as expected from its large size.

Some lanthanum(II) compounds exist but are much less stable. Therefore, in officially naming lanthanum compounds, its oxidation number must always be mentioned.

**Isotopes** Naturally occurring lanthanum consists of two isotopes: stable ¹³⁹La and primordial long-lived radioisotope ¹³⁸La. ¹³⁹La is overwhelmingly abundant, making up 99.911% of natural lanthanum; it is produced in the s-process (slow neutron capture).