How do you figure out the number of protons, electrons, and neutrons there are in an atom when you know the atomic mass?
If you know the identity of the atom, then you know the atomic number.............
And if you know the atomic number, which is the number of massive, positively charged nuclear particles, then you know the number of electrons. How do you know this? And if you know the atomic mass, then you know the number of neutrons.