How do different isotopes of the same element differ?
Isotopes differ in the number of neutrons in the nucleus
all isotopes have the same number of protons and the same number of electrons. Because the electron structure is the same isotopes have the same chemical properties.
What is different is the number of neutrons, The different number of neutrons all cause a difference in the atomic weight or mass of the atoms. The difference in the ratio between the number of protons (the number of protons stay the same) and the number neutrons creates a difference in the nuclear reactions of the isotopes.
What is different about isotopes is the number of neutrons. The difference in the number of neutrons also causes a difference in the mass and nuclear reactions of the isotopes.