In an isotope, which part of the atom changes?
When we go from one isotope to another, it is the nucleus that changes.
Isotopes are atoms of the same element with different masses.
For instance we have two forms of Carbon, Carbon-12 and
Carbon-12 has 6 protons and 6 neutrons. About 96% of Carbon on earth is Carbon-12.
Carbon-14 has 6 protons also, but has 8 neutrons, therefore making the mass 14.
Carbon-14 has a radioactive half-life of about 5780 years. It is used for carbon dating fossils from ancient living organisms.
Isotopes have varying masses because the number of neutrons is different.
The number of protons cannot be changed because the proton number defines the element.
If the electron number is different from the proton number, the particle is an ion. Extra electrons make a negative anion and fewer electrons make a positive cation.
I hope this was helpful.