Atoms having the same number of protons (and therefore the same number of electrons) but different numbers of neutrons are called isotopes of the element in question. Thus, the isotopes of an element have the same atomic number but differ in their atomic mass number.
The symbols in the adjacent example denote, respectively, the mass-235 and mass-238 isotopes of uranium (symbol U), and the mass-1,-2,and -3 isotopes of hydrogen (symbol H). The mass-2 isotope of hydrogen is also called deuterium and the mass-3 isotope is also called tritium.