All isotopes of an element share the same number of protons, which defines the element itself. However, they differ in their number of neutrons. This variance in neutron count results in different atomic masses for each isotope while retaining the same chemical properties. For instance, isotopes of hydrogen include deuterium and tritium, both having one proton but different neutron counts. Thus, the common structural feature among isotopes is their identical atomic number, signifying they belong to the same element.