Nuclear Energy
II. The Atom
The atom consists of a small, massive, positively charged core (nucleus)
surrounded by electrons (see Atom). The nucleus, containing most of the mass
of the atom, is itself composed of neutrons and protons bound together by
very strong nuclear forces, much greater than the electrical forces that
bind the electrons to the nucleus. The mass number A of a nucleus is the
number of nucleons, or protons and neutrons, it contains; the atomic number
Z is the number of positively charged protons. A specific nucleus is
designated as żU the expression ŻU, for example, represents uranium-235. See
Isotope.
The binding energy of a nucleus is a measure of how tightly its protons and
neutrons are held together by the nuclear forces. The binding energy per
nucleon, the energy required to remove one neutron or proton from a nucleus,
is a function of the mass number A. The curve of binding energy implies that
if two light nuclei near the left end of the curve coalesce to form a
heavier nucleus, or if a heavy nucleus at the far right splits into two
lighter ones, more tightly bound nuclei result, and energy will be released.
Nuclear energy, measured in millions of electron volts (MeV), is released by
the fusion of two light nuclei, as when two heavy hydrogen nuclei, deuterons
(ŞH), combine in the reaction
producing a helium-3 atom, a free neutron (¦n), and 3.2 MeV, or 5.1 ×
10-13 J (1.2 × 10-13 cal). Nuclear energy is also released when the
fission
of a heavy nucleus such as ŻU is induced by the absorption of a neutron as
in
producing cesium-140, rubidium-93, three neutrons, and 200 MeV, or 3.2 ×
10-11 J (7.7 × 10-12 cal). A nuclear fission reaction releases 10 million
times as much energy as is released in a typical chemical reaction. See
Nuclear Chemistry.
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