How did we know that the more common isotope of iron has 26 protons and 30 neutrons? I'm asking a general question so the iron example is just a special case. How did we know that oxygen has 8 protons? (and 8 neutrons), and also that the 8 neutrons is the most stable isotope and that there exists other isotopes in smaller quantities. How did we determine the quantities of the isotopes?