The number of protons in an atom is equal to the atomic number of that element. This is always true. So an atom of oxygen always has 8 protons, no matter what else about that atom is different. An atom with 11 protons is always sodium. ALWAYS.
Add the number of protons and the number of neutrons
What is the mass of the isotope of carbon with 8 neutrons?
What is the mass of the isotope of aluminum with 14 neutrons?
Electrons Negative Charge Found in energy levels outside the nucleus Mass of 1/1836 AMU (almost nothing!) The number of electrons in a neutral atom is equal to the number of protons. The number of electrons in an ion is different from the number of protons .