diatomic

from WordNet (r) 3.0 (2006)

diatomic
    adj 1: of or relating to a molecule made up of two atoms; "a
           diatomic molecule"

from The Collaborative International Dictionary of English v.0.48

diatomic \di`a*tom"ic\ (d[imac]`[.a]*t[o^]m"[i^]k), a. [Pref.
   di- + atomic.] (Chem.)
   (a) Containing two atoms.
   (b) Having two replaceable atoms or radicals.
       [1913 Webster]