<Tr> <Td_colspan="2"> <Ul> <Li> view </Li> <Li> talk </Li> <Li> </Li> </Ul> references in Wikidata </Td> </Tr> <Ul> <Li> view </Li> <Li> talk </Li> <Li> </Li> </Ul> <P> Lanthanum is a chemical element with symbol La and atomic number 57 . It is a soft, ductile, silvery - white metal that tarnishes rapidly when exposed to air and is soft enough to be cut with a knife . It is the eponym of the lanthanide series, a group of 15 similar elements between lanthanum and lutetium in the periodic table, of which lanthanum is the first and the prototype . It is also sometimes considered the first element of the 6th - period transition metals and is traditionally counted among the rare earth elements . The usual oxidation state is + 3 . Lanthanum has no biological role in humans but is essential to some bacteria . It is not particularly toxic to humans but does show some antimicrobial activity . </P> <P> Lanthanum usually occurs together with cerium and the other rare earth elements . Lanthanum was first found by the Swedish chemist Carl 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, almost three times as abundant as lead . In minerals such as monazite and bastnäsite, lanthanum composes about a quarter of the lanthanide content . It is extracted from those minerals by a process of such complexity that pure lanthanum metal was not isolated until 1923 . </P>

The element lanthanum is located at the beginning of which region
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