A centimeter-sized sample of lanthanum will corrode completely in a year as its oxide spalls off like iron rust, instead of forming a protective oxide coating like aluminium, scandium, yttrium, and lutetium. Lanthanum reacts with the halogens at room temperature to form the trihalides, and upon warming will form binary compounds with the nonmetals nitrogen, carbon, sulfur, phosphorus, boron, selenium, silicon and arsenic. Lanthanum reacts slowly with water to form lanthanum(III) hydroxide, La(OH)3. 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. Lanthanum is the strongest and hardest base among the rare earth elements, which is again expected from its being the largest of them.Some lanthanum(II) compounds are also known, but they are much less stable. Therefore, in officially naming compounds of lanthanum its oxidation number always is to be mentioned. Isotopes
Naturally occurring lanthanum is made up of two isotopes, the stable 139La and the primordial long-lived radioisotope 138La. 139La is by far the most abundant, making up 99.910% of natural lanthanum: it is produced in the s-process (slow neutron capture, which occurs in low- to medium-mass stars) and the r-process (rapid neutron capture, which occurs in core-collapse supernovae). It is the only stable isotope of lanthanum. The very rare isotope 138La is one of the few primordial odd–odd nuclei, with a long half-life of 1.05×1011 years. It is one of the proton-rich p-nuclei which cannot be produced in the s- or r-processes.
Lanthanum is relatively easy to extract as it has only one neighbouring lanthanide, cerium, which can be removed by making use of its ability to be oxidised to the +4 state; thereafter, lanthanum may be separated out by the historical method of fractional crystallization of La(NO3)3·2NH4NO3·4H2O, or by ion-exchange techniques when higher purity is desired.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:
La2O3 + 6 NH4Cl → 2 LaCl3 + 6 NH3 + 3 H2OThis is followed by reduction with alkali or alkaline earth metals in vacuum or argon atmosphere:
LaCl3 + 3 Li → La + 3 LiClAlso, pure lanthanum can be produced by electrolysis of molten mixture of anhydrous LaCl3 and NaCl or KCl at elevated temperatures. Applications
The first historical application of lanthanum was in gas lantern mantles. Carl Auer von Welsbach used a mixture of lanthanum oxide and zirconium oxide, which he called Actinophor and patented in 1886. 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.Modern uses of lanthanum include:

One material used for anodic material of nickel-metal hydride batteries is La(Ni3.6Mn0.4Al0.3Co0.7). Due to high cost to extract the other lanthanides, a mischmetal with more than 50% of lanthanum is used instead of pure lanthanum. The compound is an intermetallic component of the AB5 type.