Answer is: number of neutrons is different.
Isotopes are chemical elements with same atomic number, but different mass number (different number of neutrons).
For example, hydrogen (H), besides deuterium (mass number 2) and tritium (mass number 3), has isotope protium (mass number 1).
Atomic number is unique and defines an element. Isotopes of an element have same atomic number (same number of protons in nucleus).
Atomic number (Z) is total number of protons and mass number (A) is total number of protons and neutrons in a nucleus.
