Isotope.

Isotope (Ισότοπο) means isos (ἴσος) "equal" and topos (τόπος) "place". These are varieties of the same chemical element.

Isotopes have the same atomic number, but they have different mass numbers.

These elements are studied in chemistry.

Source of image:

https://www.shutterstock.com/ru/image-vector/representation-atom-mass-number-atomic-symbol-2154150549?trackingId=ff33b4c0-27f4-4eb2-976f-a4c89d033cc1&listId=searchResults