What do we call nuclei of the same element that have different masses?
Nuclei of the same element with different masses are called
An element is defined by
Because elements are to a first approximation electrically neutral,
Within the nucleus, a given element has a SPECIFIED number of protons, but a VARIABLE number of neutrons, massive particles of ZERO electric charge, and this gives rise to the phenomenon of isotopes. And this is best illustrated by an example.
All hydrogen nuclei have
The atomic mass quoted on the Periodic Table, is the weighted average of the masses of the isotopes. As atoms become heavier, they tend to exhibit greater isotopic variation, and again the quoted atomic mass is the envelope of the isotopic masses.