Physics for Geologists, Second edition

62 Atomic structure and age-dating
Cosmic rays ionize the atmosphere to some extent. So the ionosphere
is the part of the Earth's outer atmosphere that contains ions and free
electrons - a zone that affects radio propagation. X-rays and y-rays
ionize the material they pass through to some extent.
Isotopes and age-dating
Isotopes are species of the same chemical element that have different masses,
as we have seen. They occupy the same position in the Periodic Table because
it is the atomic number, Z, the number of protons, that determines the ele-
ment. Each has a nucleus with the same number of protons (equal to Z), but
varying numbers of neutrons. For example, the three isotopes of hydrogen
have already been mentioned: hydrogen itself (atomic weight I), deuterium
(atomic weight 2) and tritium (atomic weight 3). They each have one proton
in the nucleus but differ in the number of neutrons. Isotopes have the same
basic chemical composition, but their physical properties may differ a little.
In particular, they diffuse at different rates because diffusion depends on the
mass of the diffusing species.
Unstable isotopes differ from stable isotopes in that the change from one
to another is spontaneous. Radioactive substances change spontaneously
from the parent nuclide to daughter nuclides. In 1896 Becquerel discovered
that uranium salts affected photographic plates that were wrapped in paper,
and it is now known that uranium isotopes change spontaneously to lead
isotopes as follows:
238 92U -, 2i$~b
+ 8 ;He + 60-
Thorium also changes to an isotope of lead; and rubidium changes to an
isotope of strontium by the loss of a $-particle:
232 90Th -+ 2ii~b
+ 6 ;He + 48-
Potassium-40 decay is more complicated. It decays to both ;;Ar and to 40~a.
2P
It changes in part to argon by a reverse process, electron capture, the addition
of an electron:
!:K + e- -+ ;:Ar
but mostly to ;:Ca by $-particle emission.
Moreover, each radioisotope or radionuclide decays at a statistically con-
stant rate that is proportional to the amount of parent substance remaining
(Figures 5.1 and 5.2). It takes a fixed period of time for the parent nuclide
to convert half its atoms to a daughter nuclide; and the same time to reduce
that number of atoms to half its number; and so on. This period, known
Copyright 2002 by Richard E. Chapman