much higher than is available from the bombardment of beryllium nuclei with ( -particles. The clue to this puzzle, which Chadwick satisfactorily solved, was to assume that the neutral radiation consists of a new type of neutral particles called neutrons . From conservation of energy and momentum, he was able to determine the mass of new particle ‘as very nearly the same as mass of proton’. The mass of a neutron is now known to a high degree of accuracy.
It is m n = .00866 u = . × – kg ( . ) Chadwick was awarded the Nobel Prize in Physics for his discovery of the neutron. A free neutron, unlike a free proton, is unstable.
It decays into a proton, an electron and a antineutrino (another elementary particle), and has a mean life of about 1000s. It is, however, stable inside the nucleus. The composition of a nucleus can now be described using the following terms and symbols: Z - atomic number = number of protons [ . (a)] N - neutron number = number of neutrons [ .
(b)] A - mass number = Z + N = total number of protons and neutrons [ . (c)] One also uses the term nucleon for a proton or a neutron. Thus the number of nucleons in an atom is its mass number A. Nuclear species or nuclides are shown by the notation X A Z where X is the chemical symbol of the species.
For example, the nucleus of gold is denoted by Au . It contains nucleons, of which are protons and the rest118 are neutrons.