Atomic and Nuclear Physics
As the atomic number and the atomic mass number increase, the repulsive electrostatic forces
within the nucleus increase due to the greater number of protons in the heavy elements. To
overcome this increased repulsion, the proportion of neutrons in the nucleus must increase to
maintain stability. This increase in the neutron-to-proton ratio only partially compensates for the
growing proton-proton repulsive force in the heavier, naturally occurring elements. Because the
repulsive forces are increasing, less energy must be supplied, on the average, to remove a nucleon
from the nucleus. The BE/A has decreased. The BE/A of a nucleus is an indication of its degree
of stability. Generally, the more stable nuclides have higher BE/A than the less stable ones. The
increase in the BE/A as the atomic mass number decreases from 260 to 60 is the primary reason
for the energy liberation in the fission process. In addition, the increase in the BE/A as the atomic
mass number increases from 1 to 60 is the reason for the energy liberation in the fusion process,
which is the opposite reaction of fission.
The heaviest nuclei require only a small distortion from a spherical shape (small energy addition)
for the relatively large coulomb forces forcing the two halves of the nucleus apart to overcome
the attractive nuclear forces holding the two halves together. Consequently, the heaviest nuclei
are easily fissionable compared to lighter nuclei.
The important information in this chapter is summarized on the following page.