Fill in the Blanks ISOTOPESOnline version Definition of Isotopes by Jenny Carolina Barragán Rojas 1 protons 6 identifies element protons 13 electrons carbon different Isotopes are variants of a particular chemical which differ in neutron number , and consequently in nucleon number . All isotopes of a given element have the same number of but different numbers of neutrons in each atom . The number of protons within the atom's nucleus is called atomic number and is equal to the number of in the neutral atom . Each atomic number a specific element , but not the isotope ; an atom of a given element may have a wide range in its number of neutrons . The number of nucleons ( both protons and neutrons ) in the nucleus is the atom's mass number , and each isotope of a given element has a mass number . For example , carbon - 12 , carbon - 13 , and carbon - 14 are three isotopes of the element with mass numbers 12 , , and 14 , respectively . The atomic number of carbon is , which means that every carbon atom has 6 , so that the neutron numbers of these isotopes are 6 , 7 , and 8 respectively .