Shiny gray metal which forms a bluish oxide m.p. 921 ºC; 1690 ºF b.p. 3457 ºC; 6255 ºF density 6.145 g/cc (25 ºC); 383.62 pound/cubic foot (77 ºF)	memory peg
	1839 Carl G. Mosander, Sweden
	λανθανω (lanthanō) = to lie hidden (Greek)

Lanthanum was discovered in 1839 by Carl Gustav Mosander (1797-1858), since 1832 professor of chemistry and mineralogy at the Caroline Institute in Stockholm, as successor of Jöns Jakob Berzelius (1779-1848).

In 1825, Berzelius has asked Mosander to prepare Cerium sulphide and it was during the course of this work that Mosander became convinced that this oxide contained another earth (oxide) (Cf. Cerium). Ten years later he took up the separations again, and the existence of a new element became established in November 1838. A few months later the discovery was made more widely known by Berzelius through his letters and a detailed account in the Annual Report (Årsberättelse).

Since the new oxide was extracted as an impurity from cerium nitrate, Berzelius suggested the name lanthana, from the Greek λανθανω [lanthanō] = to lie hidden.

On 12 February 1839 Berzelius wrote Friedrich Wöhler:

"Mosander seems willing to take my suggestion to name it [the new element] Lanthanum and the oxide (the new soluble salt) lanthanum oxide or lanthana. Lanthano (Greek) means to hide or to escape notice. Lanthana lay hidden in the mineral cerite for 36 years after ceria (containing element Cerium) was discovered in the mineral cerite in 1803."

Berzelius erroneously used a τ (t) instead of a θ (th) in the Greek verb, hence Lantan instead of Lanthan.

Mosander withheld publication frustrating Berzelius and Wöhler, while continuing his efforts at purification. Finally, he announced the results of his research in a paper held in July 1842 in Stockholm. An English translaten was published in the Philosophical Magazine, and after this a German version in Poggendorf's Annalen: "Ueber die das Cerium begleitenden neuen Metalle Lanthanium und Didymium, so wie über die mit der Yttererde vorkommenden neuen Metalle Erbium und Terbium" (On the new metals Lanthanum and Didymium, accompanying Cerium, and on the metals Erbium and Terbium occuring with yttria) (note). Later, continuing analysis of Lanthana showed that it containt four new elements, see further Didymium (at Praseodymium).

For more than one century any important applications for this element were not found. The interest for lanthanum was just scientific, being limited to the investigators improving its separation techniques and purification and knowing its spectrum.

See also: Chronological list of discovery of the rare earths, their names in different languages etc. on the Yttrium page

• Mary Elvira Weeks, Discovery of the Elements, comp. rev. by Heny M. Leicester (Easton, Pa.: Journal of Chemical Education, 1968), pp. 667-699.
• Seltene Erden. Gmelins Handbuch der anorganische Chemie, 8. Aufl.; System-Nummer 39 (1938).
• Lauri Niinistö, "Discovery and Separation of Rare Earths". In Rare Earths, ed. Regino Sáez Puche & Paul A. Caro, 25-42. Madrid: Editorial Complutense, 1997.
• Lauri Niinistö, Swedish Contributions to the Discovery of Elements: Part 3: The Work of Mosander, Cleve and Nilson. ERES Newsletter, vol. 12, no. 1 (30 June 2001). (on-line).