Isotopes of an element have the same number of protons and elections but a different number of neutrons. This means there is no difference in the way that they react with oth … er elements or compounds as the electron configuration has not changed. The only difference is their new atomic mass. eg.he sum of the protons and neutr … ons is the mass number. For example, helium exists as 3He(2 protons and one neutron) or as 4He (2 protons and 2 neutrons). The two forms of helium are called isotopes of helium. Isotopes of an element have the same chemical properties but different weights.