Claude J. Allègre

Stable isotope geochemistry

When defining the properties of isotopes we invariably say that the isotopes of an element have the same chemical properties, because they have the same electron shell, but different physical properties, because they have different masses. However, if the behavior of isotopes of any chemical element is scrutinized very closely, small differences are noticeable: in the course of a chemical reaction as in the course of a physical process, isotope ratios vary and isotopic fractionation occurs. Such fractionation is very small, a few tenths or hundredths of 1%, and is only well marked for the light elements, let us say those whose atomic mass is less than 40. However, thanks to the extreme precision of modern measurement techniques, values can be measured for almost all of the chemical elements, even if they are extremely small for the heavy ones. When we spoke of isotope geochemistry in the first part of this book, we voluntarily omitted such phenomena and concentrated on isotope variations related to radioactivity, which are preponderant. We now need to look into the subtle physical and chemical fractionation of stable isotopes, the use of which is extremely important in the earth sciences. Identifying natural isotopic fractionation of light elements The systematic study of the isotopic composition of light elements in the various naturally occurring compounds brings out variations which seem to comply with a purely naturalistic logic. These variations in isotope composition are extremely slight, and are generally expressed in a specific unit, the δ unit .

Isotope Geology: Isotopes and radioactivity

Reminders about the atomic nucleus In the model first developed by Niels Bohr and Ernest Rutherford and extended by Arnold Sommerfeld , the atom is composed of two entities: a central nucleus, containing most of the mass, and an array of orbiting electrons. The nucleus carries a positive charge of + Ze , which is balanced by the electron clouds negative charge of − Ze . The number of protons, Z , is matched in an electrically neutral atom by the number of electrons. Each of these particles carries a negative electric charge e . As a rough description, the nucleus of any element is made up of two types of particle, neutrons and protons. A neutron is slightly heavier than a proton with a mass of m n = 1.674 95 · 10 −27 kg compared with m p = 1.672 65 · 10 −27 kg for the proton. While of similar masses, then, the two particles differ above all in their charges. The proton has a positive charge (+ e ) while the neutron is electrically neutral. The number of protons ( Z ) is the atomic number . The sum A = N + Z of the number of neutrons ( N ) plus the number of protons ( Z ) gives the mass number . This provides a measure of the mass of the nuclide in question if we take as our unit the approximate mass of the neutron or proton.