since the atomic mass is around 16, I figured it may be oxygen.
searched for isotopes of oxygen, found this info at isotope
Oxygen occurs in nature as 3 different isotopes, each with 8 protons. The most common isotope is 16O (8 protons, 8 neutrons), which constitutes 99.765% of all oxygen atoms on earth. There is also the rare isotope 18O (0.1995%) and the even rarer isotope 17O. The most common water molecule by far has the most common isotopes of hydrogen and oxygen ans is thus 1H216O, but there are also such molecules as H218O, and HD16O. Molecules with more than one rare, heavy isotope are really rare.