Isotopes of the same element have the same number of protons and electrons when in neutral atomic form. Different isotopes have different numbers of neutrons in their nuclei, resulting in different atomic weights for the different isotopes of a single element.
The number of protons in the nucleus of the atoms of an element gives the atomic mass of this element. Atomic mass is constant for all isotopes of the element. Because the atomic mass determines the number of electrons that must orbit the nucleus for the atom to be neutral, this atomic mass determines the chemical behavior of the element, as all non-nuclear chemistry results from the interactions of the electron clouds of atoms with one another. This means that all isotopes of the same element participate in the same way in chemical reactions.
A difference in isotope behavior arises when the nuclei of certain isotopes are unstable due to an imbalance in the number of protons to neutrons in the nucleus. There should be roughly one neutron for every proton for an atomic nucleus to be stable. As the number of neutrons increases disproportionately to the number of protons, the nucleus becomes unstable, shedding this excess neutron weight in the form of alpha and beta particles and gamma radiation.