Hi Brittany,

An isotope is basically two versions of the same kind of atom. In general, the number of protons determines the identity of the atom (something we looked at during radioactive decay) but the number of neutron present in that atom can be different. In the case of protons, it could either by 1H or 2H. 2H just has a hydrogen with an extra neutron. Since they are both the same kind of atom (have the same # of protons) but has a different molecular weight (1 vs 2 neutrons), these two atoms are called isotopes of hydrogen.

Naturally occurring isotopes are quite common. One that you might have heard about would be C-14 isotopes from Carbon-14 dating (when we try to figure out how old some really old stuff is). In general, carbon occurs as C-12, but it could have two additional neutrons to make it the isotope, C-14.

Now lets look at the question. Its asking for what a mono isotope molecule would be. The molecule in question is water. Since its asking for mono isotope, the answer would be any of the water molecules with only one of the atoms as an isotope. The best answer would be A.

Hope that helps, and happy studying!

Molly