http://www.ck12.org Chapter 22. The Nucleus
22.2 Radioactivity
- Define binding energy and its relationship to nuclear force.
- Describe nuclear disintegration.
- Define three modes of radioactive decay and write balanced nuclear equations for all three.
The machines in the image are Pressurized Water Reactors (PWRs). These are small nuclear reactors used for the
generation of power. Primarily the nuclear energy is used to change water into steam and the steam then runs the
machinery. Several hundred PWRs are used for propulsion in military vessels such as aircraft carriers, submarines,
and ice breakers. These nuclear reactors were originally designed at the Oak Ridge National Laboratory for use as
nuclear submarine power plants.
Radioactivity
The total mass of a stable nucleus is always less than the sum of the masses of its constituent protons and neutrons.
This is known asmass defect. As an example of this, consider the mass of a^42 Henucleus compared to the sum of
the masses of its constituent nucleons.
The mass of a^42 Henucleus is 4.002603 amu. The mass of two neutrons and two protons is( 2 )( 1 .008665 amu) +
( 2 )( 1 .007825 amu) = 4 .032980 amu. Thus, the mass of the helium-4 nucleus is 0.030377 amu less than the masses
of its constituents. The lost mass has gone into energy that is calledbinding energy. The binding energy is equal
to the amount of energy that would have to be put into the helium-4 nucleus in order to break it into its constituent
protons and neutrons. Without the binding energy, the repulsion between the protons would blow this nucleus apart.
We often refer to the average binding energy per nucleon which is defined as the total binding energy of a nucleus
divided by the number of nucleons in the nucleus.