Atoms of an element which have different masses are called isotopes of that
element. Since each atom of an element has the same number of
protons , it follows that the number of neutrons
must be different in different isotopes. In other words isotopes are atoms
which have the same atomic number , Z, but different
atomic mass , A.
All elements have three or more isotopes, some stable and some not. Calcium, for example, has six naturally occurring isotopes with masses of 40, 42, 43, 44, 46, and 48 which are all stable, and another 15 which are unstable.
The chemical properties of the isotopes of any one element are identical. This means that chemical processes can not be used to separate the different isotopes of a material. The physical properties of the isotopes of an element, such as mass, boiling point and freezing point are different. The nuclear properties are also very different from each other.