Explain the significance of atomic number and mass number in determining atomic properties. Which of these changes in isotopes?

Atomic Number: Number of Protons in the nucleus of the atom, also equal to the number of electrons in a neutral atom.

Mass Number: Total number of "nucleons" (Protons and Neutrons) in the nucleus

Isotopes are atoms which have the same number of protons but a different number of neutrons in the nucleus. As a result only the Mass Number would change with isotopes.