Mary

Chemist here. Of course I love all chemistry-related clues, but I take a small issue with 6-Down. The sum of protons plus neutrons is what we call the mass number and is specific to a particular isotope of an element. The atomic mass is typically understood to be a weighted average of the masses of all isotopes. The mass number is always an integer, while the atomic mass generally is not. Not trying to be a buzz kill, just taking the opportunity to educate! I managed to get the answer anyway. ;)