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
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.