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3 definitions found
 for lanthanum
From The Collaborative International Dictionary of English v.0.48 :

  Lanthanum \Lan"tha*num\ (l[a^]n"th[.a]*n[u^]m), n. [NL., fr. Gr.
     lanqa`nein to lie hid, to be concealed.] (Chem.)
     A rare element of the rare earth group of the metals, of
     atomic number 57, allied to aluminum. 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 earth elements with which it is
     usually associated. Atomic weight 138.9. Symbol La. [Formerly
     written also lanthanium.]
     [1913 Webster]

From WordNet (r) 3.0 (2006) :

      n 1: a white soft metallic element that tarnishes readily;
           occurs in rare earth minerals and is usually classified as
           a rare earth [syn: lanthanum, La, atomic number 57]

From The Elements (07Nov00) :

  Symbol: La
  Atomic number: 57
  Atomic weight: 138.9055
  (From the Greek word lanthanein, to lie hidden) Silvery metallic element
  belonging to group 3 of the periodic table and oft considered to be one
  the lanthanoids. Found in some rare-earth minerals. Twenty-five natural
  isotopes exist. La-139 which is stable, and La-138 which has a half-life
  of 10^10 to 10^15 years. The other twenty-three isotopes are
  It resembles the lanthanoids chemically. Lanthanum has a low to moderate
  level of toxicity, and should be handled with care. Discovered in 1839
  C.G. Mosander.

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