The mass number of an atom gives us the total number of protons and neutrons (in this case 19). The atomic number is number of protons as well as the number of electrons. Therefore, there must be 9 protons and electrons and 10 (19-9) neutrons.
This is the same for any atom and you can just repeat the calculation. Hope that makes sense.
Hello @hi ,
The mass number of an atom gives us the total number of protons and neutrons (in this case 19). The atomic number is number of protons as well as the number of electrons. Therefore, there must be 9 protons and electrons and 10 (19-9) neutrons.
This is the same for any atom and you can just repeat the calculation. Hope that makes sense.