Most elements have different isotopes found in nature. All isotopes of a given element have the same atomic number which is to say that they have the same number of protons in their nuclei. Assuming that the isotopes are neutral species and not charged they will all have the same number of elections as well. Where they differ is in the number of neutrons that each isotope contains in its nucleus. For example, carbon has three main isotopes: C-12, C-13, and C-14. Each isotope of carbon contains the same number of protons (6) and the same number of electrons (also 6). The mass number, or the number of protons and neutrons combined, is different for each one. C-12 has 6 neutrons, C-13 has 7 neutrons, and C-14 has 8 neutrons.