In this case, in chemistry, we defined isotopes as those atoms of the same element that eventually have the same amount of protons and electrons but different specific atomic masses which is also related with different amount of neutrons among atoms, that is why the atomic mass of an element corresponds to the average atomic mass of each isotope composing it.
Commonly, those isotopes are represented via the name of the element followed by its atomic mass, for instance carbon-14 which is widely used for dating. In such a way, for sodium-23, sodium-24 and sodium-31, we talk about the isotopes of sodium.
