How does the atomic mass of the atom change when an electron is added or removed?

If you could magically make two neutrons in the nucleus of an atom disappear, the atomic number and the electrical charge of that atom would remain unchanged. Neutrons do not carry an electrical charge so adding or removing them from the nucleus does not change the electrical charge of the nucleus. It does, however, change the mass of the nucleus. Adding or removing neutrons from the nucleus are how isotopes are created.

Protons carry a positive electrical charge and they alone determine the charge of the nucleus. Adding or removing protons from the nucleus changes the charge of the nucleus and changes that atom's atomic number. So, adding or removing protons from the nucleus changes what element that atom is! For example, adding a proton to the nucleus of an atom of hydrogen creates an atom of helium. (Actually, a few neutrons have to be added as well to make the new nucleus stable, but the end result is still helium.) Keep in mind that atoms, by definition, are electrically neutral and always contain the same number of protons and electrons.

i'm pretty sure it is letter c.) car