Isotopes are distinct nuclear species of the same chemical element.
Nuclides having the same atomic number but different mass numbers
Any of two or more atoms with the same atomic number that contain different numbers of neutrons.
Any of two or more forms of an element where the atoms have the same number of protons, but a different number of neutrons within their nuclei. As a consequence, atoms of isotopes will have the same atomic number but a different mass number.