The isotopes of the same element only have a different number of neutrons. Since neutrons have a very negligible influence on the size of the atoms, the isotopes of the same element have almost identical radii.
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Conversely, changes in number of electrons amongst the ions of the same element make a huge difference. Since, with change in number of electrons, relative charges between the core and the shell will be different, the strength of the interactions between the core and the shell will differ also, as well as amongst the electrons themselves. Therefore, if you add electrons to an atom to form an anion, its size will increase due to the greater repulsion amongst the electrons, while if you take electrons to form a cation, the remaining electrons will be more strongly pulled by the core, and the size of the cation will decrease.