Different isotopes of an element have different numbers of neutrons.
Explanation:
There are various elements existing in the Universe. Each element is made up of atoms and there can be multiple types of atoms of an element based on the numbers and configurations of its sub-particles i.e. protons, electrons and neutrons. These are known as Isotopes.
Isotopes are the specific set of atoms of an element that have same proton and electron counts but different neutron counts. The isotopes have same protons and electrons count, thus having the same atomic number but, different atomic mass number because of different number of neutrons.