Isotope
In any neutral atom, the number of electrons is the same as the number of protons. As a result, isotopes of the same element also have the same number of electrons and the same electronic structure. The way an atom acts is decided by its electronic structure, so isotopes of the same element have almost the same chemical behavior, such as the molecules it can form. It is very difficult to separate the isotopes of an element from a mixture of different isotopes because of how similar this behavior is.