Isotopes are different ‘versions’ of atoms of the same element. Isotopes of an element differ from each other only in the number of neutrons in the nucleus (particles of one atomic mass unit, but no electric charge).
The difference in the number of neutrons affects the weight (correctly called ‘mass’) of the atom. While the electronic charge of the atom, and the way that it bonds to other atoms is unchanged, the slight difference in weight causes the bonds that it makes to be just a tiny bit stronger. It’s this tiny difference that leads to fractionation between isotopes in molecules whenever any sort of reaction occurs.
Some reactions that lead to fractionation include:
- evaporation and condensation