[Harvest72]

Thanks for the reply,

That makes sense. So if we look at an atom of Nitrogen, the only reason it is not exactly 14 amu is because of the extra electron?

I'm confused about this question:

Gallium consists of two naturally ocouring isotopes with masses of 68.926 and 70.925 amu. How many protons and neutrons are in the nucleus of each isotope?

Well of course there is 31 protons in each, but how do you know for sure how many neutrons there are? Looking at the answers it would appear that you just round off to the nearest whole number and subtract the protons. This makes sense I guess but why the decimals?

Thanks,
Cam