The Collaborative International Dictionary of English v.0.48:

Monatomic \Mon`a*tom"ic\, adv. [Mon- + atomic.] (Chem.)
   (a) Consisting of, or containing, one atom; as, the molecule
       of mercury is monatomic.
   (b) Having the equivalence or replacing power of an atom of
       hydrogen; univalent; as, the methyl radical is monatomic.
       [1913 Webster]

WordNet (r) 3.0 (2006):

monatomic
    adj 1: of or relating to an element consisting of a single atom;
           "helium and argon are monatomic gases" [syn: monatomic,
           monoatomic]