LANTHANUM

lanthanum, La, atomic number

(noun) a white soft metallic element that tarnishes readily; occurs in rare earth minerals and is usually classified as a rare earth

Source: WordNet® 3.1

Etymology

Noun

lanthanum (countable and uncountable, plural lanthanums)

(uncountable) A chemical element (symbol La) with an atomic number of 57: a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air.

(countable) An atom of this element.

Source: Wiktionary

Lan"tha*num, n. Etym: [NL., fr. Gr. (Chem.)

Definition: A rare element of the group of the earth metals, allied to aluminium. It occurs in certain rare minerals, as cerite, gadolinite, orthite, etc., and was so named from the difficulty of separating it from cerium, didymium, and other rare elements with which it is usually associated. Atomic weight 138.5. Symbol La. [Formerly written also lanthanium.]

Source: Webster’s Unabridged Dictionary 1913 Edition