Which isotope has 16 neutrons and is used to study nucleotides and nucleic acids? a. phosphorus-31 b. phosphorus-32 c. sulfur-32 d. sulfur-33

When naming isotopes, the number beside it indicates the mass number. Now mass number is the sum of protons and neutrons. 

We know that the mass number of a stable Sulfur atom is 32, so  we can rule out C as an isotope. The atomic number of Sulfur is 16 and that means that there are 16 protons. Now if we subtract 16 from 33, then that means there are 17 neutrons. So we can rule out D. 

Phosphorus on the other hand has an atomic number of 15. If we subtract 15 from 31 we will have 16. That means that Phosphorus-31 has 16 neutrons.

The answer would then be A. Phosphorus 31.