Physics for Geologists, Second edition

60 Atomic structure and age-dating
repel. So the electrons are attracted to the nucleus but repel each other.
Moving charges generate a magnetic field.
Particles have energy, and the unit is the electron volt, eV [ML~T-~]. This
is the energy acquired by a particle carrying a single charge when acted on
by a potential difference of one volt. 1 eV = 160.210 x joules (J). Be
careful with units and dimensions here because the symbol is to be seen as
a single symbol: eV, not e x V. An electric charge in coulombs [As] acted
on by a volt [W A-l] gives an energy in watt-seconds which equals joules
[W = J s-'1. The charge on an electron (e) is a fundamental and natural unit
of electricity. Its value is 160.210 x lop2' coulombs (C), and all non-zero
charges are a whole multiple of this.
The number of protons is called the atomic number (Z, which is dimensionless).
Elements are placed in the Periodic Table according to their atomic
number because an element is determined by the charge of the protons in
the nucleus, not by the neutrons. Neutrons and protons are much more
massive than electrons (by factors of 1836.1 and 1838.63, respectively),
and the total number of neutrons (N) and protons (Z) is called the atomic
mass number (A). This is a whole number, dimensionless, that is very nearly
equal to the mass of the atom because of the small mass of the electrons.
The full notation is given by prefixes to the abbreviation of the element; the
atomic number written subscript, and the atomic mass number written superscript,
$. For example, '$ means that there are 6 protons, and 12 protons
and neutrons (so, 6 neutrons) in the nucleus. The symbol is often abbreviated
by leaving out the atomic number, for example, 12C. In older works
you may find the superscript prefix after the element abbreviation, as c14.
Isotopes
Some atoms of simple elements have differing numbers of neutrons. Hydrogen
is found in three forms. :H,:H and :H, which we call hydrogen,
deuterium and tritium. These are isotopes of hydrogen. Another example
is carbon, which also has three isotopes: ';c, ' 2 ~
and *:c. Isotopes of an
element have the same general chemical properties (because protons, not
neutrons, determine this) but differ slightly in mass and in some physical
properties. They occupy the same place in the Periodic Table because they
have the same atomic number.
Isotopes are of two sorts: stable and unstable. Certain combinations of
neutrons and protons are stable, others unstable. The unstable isotopes
change spontaneously to a more stable form by emitting or capturing parti-
cles. These unstable isotopes are also called radioisotopes. It turns out that
stable isotopes are those in which Z and N are nearly equal (or Z is about
half A) - a small minority of the isotopes. Tritium, :H, is an unstable isotope
of hydrogen. The decay of an unstable isotope also involves the emission of
quanta of energy. The use of radioisotopes, such as ':c, in age-dating will
be discussed below.
Copyright 2002 by Richard E. Chapman