isotope

One of two or more atoms having the same atomic number but different mass numbers.

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 for the same isotope will have the same atomic number but a different mass number (atomic weight).

To define or demonstrate an isotopy of (one map with another).

one of two or more atoms with the same atomic number but with different numbers of neutrons