** In a chemical reaction there is merely a rearrangement of extranuclear electrons. The atomic nucleus remains intact.
** A nuclear reaction involves a change in the composition of the nucleus. The number of protons and neutrons in the nucleus is altered. The product is a new nucleus of another atom with a different atomic number and/or mass number. Thus:
Nuclear reaction: is one which proceeds with a change in the composition of the nucleus so as to produce an atom of a new element.
** The conversion of one element to another by a nuclear change is called Transmutation.
** We have already considered the nuclear reactions of radioactive nuclei, producing new isotopes.
** Here we will consider such reactions caused by artificial means.
Differences between Nuclear Reactions and Chemical Reactions
Nuclear Fission Reactions
** In these reactions an atomic nucleus is broken or fissioned into two or more fragments.
** This is accomplished by bombarding an atom by alpha particles (4He2 ), neutrons (1n0), protons (1H1), deutrons (2H1), etc.
** All the positively charged particles are accelerated to high kinetic energies by a device such as a cyclotron. This does not apply to neutrons which are electrically neutral.
** The projectile enters the nucleus and produces an unstable ‘compound nucleus’. It decomposes instantaneously to give the products.
** For example, 14N7when struck by an α-particle first forms an intermediate unstable compound nucleus, 18F9 , which at once cleaves to form stables 17O8
** Other examples are:
** Nuclear fission reactions are classified according to the projectile used and the particle that is emitted. In Figure above the type of the reaction has been stated.
** It is noteworthy that neutrons are particularly useful as projectile. Sir James Chadwick obtained these by bombarding beryllium-9 with α-particles. Being electrically neutral, neutrons pierce the positive nucleus easily.
Nuclear Fusion Reactions
** These reactions take place by combination or fusion of two small nuclei into a larger nucleus.
** At extremely high temperatures the kinetic energy of these nuclei overweighs the electrical repulsions between them. Thus they coalesce to give an unstable mass which decomposes to give a stable large nucleus and a small particle as proton, neutron, positron, etc.
** For example :
(1) Two hydrogen nuclei, 1H1, fuse to produce a deuterium nucleus, 2H1.
(2) Deuterium nucleus, 2H1, and tritium nucleus, 3H1 , combine to give helium nucleus, 4He2 with the expulsion of a neutron.
Differences between Nuclear Fission and Nuclear Fusion
Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.