http://www.ck12.org Chapter 25. Nuclear Physics
25.3 Nuclear Fission and Fusion
Vocabulary
- nuclear fission:The splitting of an atom with a large atomic mass into at least two relatively equal parts, with
an accompanying release of energy. - nuclear fusion:The process in which the nuclei of lighter elements combine and form heavier elements, with
an accompanying release of energy.
Nuclear fission
Earlier, we saw that high-energy neutrons were able to change nitrogen into radiocarbon. Physicists have known
since 1919 that other natural processes also caused the transmutation of elements. (Note: Transmutation occurs in
both nuclear fission and nuclear fusion reactions.) In 1919, Ernest Rutherford observed that, as high-energy alpha
particles passed through nitrogen gas, oxygen atoms and free protons(p)were emitted. He reasoned that the nitrogen
nuclei had taken on one additional proton and two additional neutrons, and thus turned into oxygen-17.
The transformation can be written as:
4
2 He+
14
7 N→17
8 O+p
Rutherford’s experiment was the first instance of what is called the artificial transmutation of elements. During the
1930s, the famous Italian physicist Enrico Fermi (1901-1954), began experimenting in that field. He realized that
slow-moving neutrons would be the best particles to use in inducing nuclear reactions, because they are not repelled
by the nucleus. In fact, if they got close enough to the nucleus, they would be attracted by the strong nuclear force
and captured by nuclei.
Fermi’s idea proved correct. Bombarding the heaviest known naturally occurring element, uranium,Z=92 with
neutrons, he was able to produce the first transuranic (beyond uranium) elements that are not found anywhere on
Earth. Examples include Neptunium(Ne,Z= 93 )and Americium(Am,Z= 95 ).
Two German scientists, Otto Hahn and Fritz Strassmann, built on Fermi’s work in the late 1930s. They noticed that
at times, when uranium was exposed to slow neutrons, two smaller nuclei, roughly half the size of the uranium atom,
emerged from the reaction. How could this be? Up until that time, only small particles such as alpha particles were
seen to be ejected in such reactions.
A former Jewish colleague of Hahn’s, Lise Meitner (1878-1968) and her nephew and colleague, Otto Frisch (1904-
1979) had left Nazi Germany fearing the anti-Semitism of Hitler’s Germany, and were working in Sweden at the
time.
When Hahn reported the peculiar results of the experiment to Meitner (who shared them with Frisch), they quickly
realized what had occurred. The captured neutron increased the internal energy of the nucleus of the uranium
atom,Figure25.6. The additional energy caused an increase in the motion of the nucleons in the atom. That, in
turn, caused the nucleus to expand. As the nucleus expanded, the strong nuclear force weakened, permitting the
electrostatic repulsion to split the uranium atom into nearly two equal parts,N 1 and N 2. Hahn and Strassmann had
spilt the atom.
The splitting of an atom with a large atomic mass into at least two relatively equal parts, with an accompanying
release of energy, is called nuclear fission.