All the isotopes of an element have the same number of protons in the nucleus BUT they have different numbers of neutrons in the nucleus. For example;
carbon-12 has 6 protons and 6 neutrons in the nucleus of the atom
carbon-13 has 6 protons and 7 neutrons in the nucleus of the atom
carbon-14 has 6 protons and 8 neutrons in the nucleus of the atom
