Lanthanum is a soft, ductile, silvery-white metallic chemical element with symbol La and atomic number 57. It tarnishes rapidly when exposed to air and is soft enough to be cut with a knife. It gave its name to the lanthanide series, a group of 15 similar elements between lanthanum and lutetium in the periodic table. It is also sometimes considered the first element of the 6th-period transition metals, and is traditionally counted among the rare earth elements. As such, it almost always assumes the oxidation state +3. Lanthanum has no biological role and is not very toxic.
Lanthanum usually occurs together with cerium and the other rare earth elements. Lanthanum was first found by the Swedish chemistCarl Gustav Mosander in 1839 as an impurity in cerium nitrate – hence the name lanthanum, from the Ancient Greek λανθάνειν (lanthanein), meaning "to lie hidden". Although it is classified as a rare earth element, lanthanum is the 28th most abundant element in the Earth's crust, being just under three times as abundant as lead. In minerals such as monazite and bastnäsite, lanthanum makes up about a quarter of the lanthanide content. It is extracted from these minerals using a process of such complexity that pure lanthanum metal was not isolated until 1923.
Lanthanum compounds have numerous applications as catalysts, additives in glass, carbon lighting for studio lighting and projection, ignition elements in lighters and torches, electron cathodes,scintillators, GTAW electrodes, and others. Lanthanum carbonate has been approved as a medicine for treating renal failure.