Protons, neutrons and electrons in ions

The atomic number of an element is its number of protons. The mass number is the total number of protons and neutrons, so the number of neutrons is the mass number minus the atomic number. When an atom forms an ion, only the electrons change. The numbers of protons and neutrons stay the same.

For a positive ion, the atom has lost electrons, so the number of electrons is the atomic number minus the charge. A sodium ion, Na⁺, has atomic number 11 and mass number 23. It has 11 protons, 12 neutrons and 10 electrons.

For a negative ion, the atom has gained electrons, so the number of electrons is the atomic number plus the size of the charge. A chloride ion, Cl^-, has atomic number 17 and mass number 35. It has 17 protons, 18 neutrons and 18 electrons. An oxide ion, O^2-, has atomic number 8 and mass number 16. It has 8 protons, 8 neutrons and 10 electrons.