isotopes

/AY1 S AH0 T OW0 P S/
noun
  1. 1

    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.

Source: https://en.wiktionary.org/wiki/isotope, https://en.wiktionary.org/wiki/isotopes