What are Natural Isotopes?
Isotopes are atoms of the same element with different masses. They get
these different masses by having different numbers of neutrons in
their nucleii. They are the same type of atom, however, because
their nucleii have the same number of protons in them.
Isotopes of atoms that occur in nature come in two flavors: stable and unstable (radioactive). Some of the unstable isotopes are only moderately unstable and can therefore still persist in nature today. The isotope 238-U is a good example. It is radioactive but it's half life is 4.43 billion years. The Earth itself is 4.55 billion years old, so we now have roughly half of the 238-U on Earth that we had when Earth was formed. When an unstable isotope decays, it makes a new atom of a different element. Stable, isotopes, on the other hand, do not decay.
What determines whether an isotope is stable or unstable, and if it is unstable, how unstable it is (i.e., how short it's half life is) depends on the energy of the configuration of that particular nucleus. This is a subject of nuclear physics and too detailed to go into here.
Just so you know, there are also non-Natural (man made) isotopes. These are all radioactive.
Dr. Ken Rubin, Assistant Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822