I do not believe "elementary molecule" is still a common term in chemistry, but I did find the following wikipedia article that explains that Avogadro used to refer to atoms as "elementary molecules" and to compounds as "compound molecules" (http://en.wikipedia.org/wiki/History_of_molecular_theory#19th_century). If any other tutors know of a more recent distinction here, that would be appreciated.
Molecules are substances that are comprised of more than one atom. These atoms may be joined by ionic bonds or covalent bonds. Compounds, specifically, are molecules that combine more than one element; this excludes substances such as oxygen, hydrogen, and nitrogen, all of which naturally exist in a molecular form: O2, H2, and N2.
I hope this helps. Please let me know if you have any other questions.