Isotopes of a particular element are all of the same element even though their mass number changes.
All atoms of a particular element have exactly the same number of protons regardless of their mass number. It is the number of protons that define what the element is.
The reason why the mass number changes for various isotopes of the same element is because the number of neutrons changes. Therefore as the mass number increases the number of neutrons changes but the number of protons has to remain constant.