Source : Webster's Revised Unabridged Dictionary (1913)

Diatomic \Di`a*tom"ic\, a. [Pref. di- + atomic.] (Chem.)
      (a) Containing two atoms.
      (b) Having two replaceable atoms or radicals.

Source : WordNet®

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