The textbook explains that the atomic number is defined by the number of protons, which remains 12. The atomic mass will increase because normally, 'the electron... has almost negligible mass, and hence, can be ignored in calculations,' but a particle 500 times heavier adds substantial mass. The mass number remains unchanged because 'Mass number = Number of protons + Number of neutrons,' and neither nucleon count changes. The overall charge remains neutral because 'for an atom to be electrically neutral, the number of protons must be equal to the number of electrons' (or equivalent negatively charged particles).