The Atom
All
matter is composed of individual entities called elements. Each element is
distinguishable from the others by the physical and chemical properties of its
basic component—the atom. Originally thought to be the “smallest” and
“indivisible” particle of matter, the atom is now known to have a substructure
and can be “divided” into smaller components. Each atom consists of a small
central core, the nucleus, where most of the atomic mass is located, and a
surrounding “cloud” of electrons moving in orbits around the nucleus. Whereas
the radius of the atom (radius of the electronic orbits) is approximately 10 -10
m, the nucleus has a much smaller radius, namely about 10 -15 m.
Thus, for a particle of size comparable to nuclear dimensions, it will be quite
possible to penetrate several atoms of the matter before a collision happens. It is
important to keep track of those particles that have not interacted with the
atoms (the primary beam) and those that have suffered collisions (the scattered
beam).
The Nucleus
The
properties of atoms are derived from the constitution of their nuclei and the
number and organization of the orbital electrons. The nucleus contains two
kinds of fundamental particles: protons and neutrons. Whereas protons are
positively charged, neutrons have no charge. Because the electron has a
negative unit charge (1.60 × 10 -19 Coulombs) and the proton has a
positive unit charge, the number of protons in the nucleus is equal to the
number of electrons outside the nucleus, thus making the atom electrically
neutral. An atom is completely specified by the formula AZX,
where X is the chemical symbol for the element; A is the mass number, defined
as the number of nucleons (neutrons and protons in the nucleus); and Z is the
atomic number, denoting the number of protons in the nucleus (or the number of
electrons outside the nucleus). An atom represented in such a manner is also
called a nuclide. For example, 11H and 42He
represent atoms or nuclei or nuclides of hydrogen and helium, respectively.
On
the basis of different proportions of neutrons and protons in the nuclei, atoms
have been classified into the following categories: isotopes, atoms having
nuclei with the same number of protons but a different number of neutrons;
isotones, having the same number of neutrons but a different number of protons;
isobars, with the same number of nucleons but a different number of protons; and
isomers, containing the same number of protons as well as neutrons. The last
category, namely isomers, represents identical atoms except that they differ in
their nuclear energy states. For example, 131µ54Xe (µ
stands for metastable state) is an isomer of 13154Xe.
Certain combinations of neutrons and protons result in more stable
(nonradioactive) nuclides than others. For instance, stable elements in the
low-atomic-number range have an almost equal number of neutrons, N, and
protons, Z. However, as Z increases beyond about 20, the neutron-to-proton
ratio for stable nuclei becomes greater than 1 and increases with Z.
Nuclear
stability has also been analyzed in terms of even and odd numbers of neutrons
and protons. Of about 300 different stable isotopes, more than half have even
numbers of protons and neutrons and are known as even-even nuclei. This
suggests that nuclei gain stability when neutrons and protons are mutually
paired. On the other hand, only four stable nuclei exist that have both odd Z
and odd N, namely 21H, 63Li, 105B,
and 147N. About 20% of the stable nuclei have even Z and
odd N and about the same proportion have odd Z and even N.