8.2.8 Nuclear energy and binding energies
In the same way that chemical reactions can be classified as
exothermic (releasing energy) or endothermic (requiring energy to
react), so nuclear reactions may either release or use up energy. The
energies involved in nuclear reactions are greater by a huge factor.
Thousands of tons of coal would have to be burned to produce as
much energy as would be produced in a nuclear power plant by one
kg of fuel.
Although nuclear reactions that use up energy (endothermic
reactions) can be initiated in accelerators, where one nucleus is
rammed into another at high speed, they do not occur in nature, not
even in the sun. The amount of kinetic energy required is simply
not available.
To find the amount of energy consumed or released in a nuclear
reaction, you need to know how much nuclear interaction energy,
Unuc, was stored or released. Experimentalists have determined the
amount of nuclear energy stored in the nucleus of every stable el-
ement, as well as many unstable elements. This is the amount of
mechanical work that would be required to pull the nucleus apart
into its individual neutrons and protons, and is known as the nuclear
binding energy.
A reaction occurring in the sun example 8
The sun produces its energy through a series of nuclear fusion
reactions. One of the reactions is(^1) H + (^2) H→ (^3) He +γ
The excess energy is almost all carried off by the gamma ray (not
by the kinetic energy of the helium-3 atom). The binding energies
in units of pJ (picojoules) are:
(^1) H 0 J
(^2) H 0.35593 pJ
(^3) He 1.23489 pJ
The total initial nuclear energy is 0 pJ+0.35593 pJ, and the final
nuclear energy is 1.23489 pJ, so by conservation of energy, the
gamma ray must carry off 0.87896 pJ of energy. The gamma ray
is then absorbed by the sun and converted to heat.
self-check E
Why is the binding energy of^1 H exactly equal to zero? .Answer, p.
1058
Figure s is a compact way of showing the vast variety of the
nuclei. Each box represents a particular number of neutrons and
protons. The black boxes are nuclei that are stable, i.e., that would
require an input of energy in order to change into another. The
516 Chapter 8 Atoms and Electromagnetism