Under science category:
An element of different molecular mass.
A form of an element of a specific atomic mass.
The number of neutrons varies between isotopes. Like this some elements have different atomic masses.
For example: carbon-12 has six protons and six neutrons, whereas carbon-14 has 6 protons and 8 neutrons. These are different isotopes of the same element.
Note: In The Simpsons, the Springfield baseball team is called The Springfield Isotopes. Echoing Homer's job in the nuclear power plant.