Theory of Isotopes

Abstract

The very first thing that we have to examine is the nature of isotopes: what they are, when and how they were formed, and what their characteristics are. The word isotope itself means “same place”, from the Greek words ισοσ (“same”) and τοποσ (“place”), and refers to their position in the periodic table of the elements. Here, we will discuss how isotopes are affected by various physical and chemical processes and the passage of time.

Further Reading

Many books describe the basic principles of isotopes and isotope fractionation, some in greater detail, others in terms easier to understand. Here is a short selection of these:

• Principles of Stable Isotope Distribution, R.E. Criss, Oxford University Press, 1999, ISBN 978-0-19-511775-2. Of greatest use for our purposes here are Chapters 1, 2, and 4, as well as Appendix A3.

• Stable Isotope Geochemistry, J. Hoefs, Springer-Verlag, 1997, ISBN 3-540-61126-6. Chapters 1 and 2 are the most significant to us.

• The Elements, P.A. Cox, Oxford University Press, 1990, ISBN 0-19-855298-X. A very interesting little book, the most useful chapters being 1, 3, and 6.

• Stable Isotope Geochemistry: a Tribute to Samuel Epstein, H.P. Taylor, J.R. O’Neil, and I.R. Kaplan, Editors. The Geochemical Society Special Publication #3, ISBN 0-941809-02-1. Of particular interest to us is Part A, which discusses isotopic fractionations.

• Isotopes: Principles and Applications, G. Faure and T.M. Mensing, Wiley, 2005, ISBN 978-047-138437-3.

Questions

• What are isotopes defined by? What is the single most important characteristic of an isotope? What is the difference between the different isotopes of the same chemical element?

• When, where, and how are isotopes formed?

• What is the line of stability? Why is it positioned in its particular location? What happens to isotopes that are situated away from it?

• How do we calculate isotopic composition? Relative to what do we calculate it?

• What is fractionation? What types of fractionation are there and what factors affect them? How do we calculate fractionation?

• What is radiogenic disintegration? What modes of disintegration and what types of radiation are there?