In chemical reactions, lanthanum almost always gives up these three valence electrons from the 5d and 6s subshells to form the +3 oxidation state, achieving the stable configuration of the preceding noble gas xenon.

)[24] This chemical availability of f orbitals justifies lanthanum's placement in the f-block despite its anomalous ground-state configuration[25][26] (which is merely the result of strong interelectronic repulsion making it less profitable to occupy the 4f shell, as it is small and close to the core electrons).

[33] In dilute sulfuric acid, lanthanum readily forms the aquated tripositive ion [La(H2O)9]3+: This is colorless in aqueous solution since La3+ has no d or f electrons.

When it reacts with water, lanthanum hydroxide is formed:[37] a lot of heat is evolved in the reaction and a hissing sound is heard.

[38] Lanthanum reacts exothermically with hydrogen to produce the dihydride LaH2, a black, pyrophoric, brittle, conducting compound with the calcium fluoride structure.

[38] Due to the large ionic radius and great electropositivity of La3+, there is not much covalent contribution to its bonding and hence it has a limited coordination chemistry, like yttrium and the other lanthanides.

Oxygen is the most common donor atom in lanthanum complexes, which are mostly ionic and often have high coordination numbers over 6 : 8 is the most characteristic, forming square antiprismatic and dodecadeltahedral structures.

These high-coordinate species, reaching up to coordination number 12 with the use of chelating ligands such as in La2(SO4)3· 9(H2O), often have a low degree of symmetry because of stereo-chemical factors.

The best characterized organolanthanum compounds are the cyclopentadienyl complex La(C5H5)3, which is produced by reacting anhydrous LaCl3 with NaC5H5 in tetrahydrofuran, and its methyl-substituted derivatives.

[41] In 1751, the Swedish mineralogist Axel Fredrik Cronstedt discovered a heavy mineral from the mine at Bastnäs, later named cerite.

Thirty years later, the fifteen-year-old Wilhelm Hisinger, from the family owning the mine, sent a sample of it to Carl Scheele, who did not find any new elements within.

In 1803, after Hisinger had become an ironmaster, he returned to the mineral with Jöns Jacob Berzelius and isolated a new oxide which they named ceria after the dwarf planet Ceres, which had been discovered two years earlier.

[44][45] He partially decomposed a sample of cerium nitrate by roasting it in air and then treating the resulting oxide with dilute nitric acid.

[b][47] That same year, Axel Erdmann, a student also at the Karolinska Institute, discovered lanthanum in a new mineral from Låven island located in a Norwegian fjord.

Since lanthanum's properties differed only slightly from those of cerium, and occurred along with it in its salts, he named it from the Ancient Greek λανθάνειν [lanthanein] (lit.

[50] The La3+ ion is similarly sized to the early lanthanides of the cerium group (those up to samarium and europium) that immediately follow in the periodic table, and hence it tends to occur along with them in phosphate, silicate and carbonate minerals, such as monazite (MIIIPO4) and bastnäsite (MIIICO3F), where M refers to all the rare earth metals except scandium and the radioactive promethium (mostly Ce, La, and Y).

The ore, after being crushed and ground, is first treated with hot concentrated sulfuric acid, evolving carbon dioxide, hydrogen fluoride, and silicon tetrafluoride: the product is then dried and leached with water, leaving the early lanthanide ions, including lanthanum, in solution.

[52] Lanthanum metal is obtained from its oxide by heating it with ammonium chloride or fluoride and hydrofluoric acid at 300–400 °C to produce the chloride or fluoride:[17] This is followed by reduction with alkali or alkaline earth metals in vacuum or argon atmosphere:[17] Also, pure lanthanum can be produced by electrolysis of molten mixture of anhydrous LaCl3 and NaCl or KCl at elevated temperatures.

The original mantles gave a green-tinted light and were not very successful, and his first company, which established a factory in Atzgersdorf in 1887, failed in 1889.

The lanthanum cation La3+ is a positive allosteric modulator at native and recombinant GABA receptors, increasing open channel time and decreasing desensitization in a subunit configuration dependent manner.

[75] Lanthanum is a cofactor for the methanol dehydrogenase of the methanotrophic bacterium Methylacidiphilum fumariolicum SolV, although the great chemical similarity of the lanthanides means that it may be substituted with cerium, praseodymium, or neodymium without ill effects, and with the smaller samarium, europium, or gadolinium giving no side effects other than slower growth.

The injection of lanthanum solutions produces hyperglycemia, low blood pressure, degeneration of the spleen and hepatic alterations.

[80] Lanthanum meant for ingestion, typically as a chewable tablet or oral powder, can interfere with gastrointestinal (GI) imaging by creating opacities throughout the GI tract; if chewable tablets are swallowed whole, they will dissolve but present initially as coin-shaped opacities in the stomach, potentially confused with ingested metal objects such as coins or batteries.