An Isotope

1. (physics) 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.

Usage notes[edit]

Technically, isotopes are nuclides having the same atomic number but different mass number. In practice, the term isotope is often used instead of nuclide.