Answer is the mass of the atom. Protons + neutrons= mass Let's say you have oxygen with a mass of 16. Oxygen has 8 protons To find neutrons subtract 16-8= 8 So it would also be 8 neutrons. Example of how to find mass if you know protons + neutrons: Sodium has 11 protons and 12 neutrons. Add the protons and neutrons 11 + 12= 23 Mass is 23
