are two or
more forms of the same
element having the same
atomic number (number of protons) but different number of
neutrons and thus, different having different nucleon number.
Examples of isotopes of carbon are carbon-12 (C-12), carbon-13
(C-13) and carbon-14 (C-14). All these isotopes have the same
number of protons (that is 6 because they are all the same element
carbon). The three isotopes, however, have 6, 7 and 8 neutrons in
their nucleus respectively.