b. Using this notation, an atom can be easily described. For
example, the atomic notation at the right is for an atom with 92 protons
(uranium--symbol U), 92 electrons in shells around the nucleus, and a total
of 235 nucleons in the nucleus. Since 92 of the nucleons are protons, there
are 143 neutrons in the nucleus of this atom.
a. It was indicated earlier that the negative electrons are bound to the atom by
the attraction of the positive protons. Within the nucleus, however, only positive
charges exist and these charges repel each other. Therefore, other forces strong
enough to overcome the repelling forces of the protons must exist between nucleons.
Since these forces are of very short range, acting only between nucleons close to one
another, they are called strong nuclear forces. It is possible in isotopes of heavier
elements for the electrostatic forces between protons to overcome these strong nuclear
forces. If this is the case, part of the nucleus actually may break off and escape. In
other cases, rearrangements may take place, which lead to more stable configurations
within the nucleus (see para e below). Nuclei in which this happens are said to be
unstable or radioactive. All isotopes with atomic number Z greater than 83 are naturally
radioactive and many more isotopes can be made artificially radioactive by adding
neutrons, protons, or groups of these to destabilize the normally stable configuration.
b. Natural radioactivity occurs in three basic ways. Many unstable nuclei emit a
particle composed of two protons and two neutrons called an alpha particle (symbol α).
This configuration is also the nucleus of a helium (He) atom and is very stable. To see
what results from an emission, consider the isotope of uranium with 146 neutrons and
92 protons (U-238). This is a naturally-occurring isotope, but is radioactive and an
alpha emitter (Figure 1-6). Since emission removes the two protons, the uranium
changes to an entirely new substance, thorium, in the form of the thorium isotope with
90 protons and 234 nucleons (protons + neutrons).
Figure 1-6. Alpha emission from U-238.