Isotopes are elements in the periodic table that have the same atomic number, but a different mass number.
13 C6 12 C6
Whats the Difference?
The number of protons determine the Atomic Number in an element. The atomic number never changes. Only the number of neutrons can change.
Electrons and Protons need to have an equal amount. Electrons need to have a equal amount to make the element stable, not unstable and to have a neutral charge. They do not affect the atomic mass since they weigh much less than neutrons and protons.