The atomic weight of an element in nature is the weighted average, in terms of natural abundance, of the atomic weights of its naturally occurring isotopes. Boron is a good example since it has appreciable amounts of two stable isotopes.

Because isotopes of the same element have such similar chemical properties, the ratio of isotopes ordinarily is unchanged during chemical reactions.

If individual isotopes are wanted, they must be separated by some technique, such as diffusion or mass spectrometry, that is sensitive to small mass differences.

To the chemist, all isotopes of an element react in much the same way. What is important to chemical behavior is not the number of neutrons in an atom, but the number of protons because this determines the number of electrons, and electrons give rise to all of the important chemical properties of the elements.