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The classical atom comprises Protons, Neutrons, and Electrons. The atom has a number of
protons and neutrons bonded using nuclear forces in the central nucleus. The atomic number
of an element is the number of protons in the nucleus of its atoms. The atomic number defines
its chemical properties.
The mass of a proton mp = 1.673 x 10 -27 kg = 1.007277 u
The mass of an atom is always less than the sum of the masses of the neutrons, protons and electrons. It has a mass defect.. The energy equivalent of the missing mass is called the binding energy the higher the binding energy the more stable the atom. The binding energy is effectively the energy released when an atom is formed from its elemental particles. The mass defect of a nucleus with Z atoms and N neutrons is calculated from its atomic mass m as follows
Dm = Z . mH + N . mn - m
mH = the mass of the Hydrogen atom = 1.07825 u
To calculate the energy in Mega-electronvolt Dm is multiplied by
Nuclear reactions include nuclear fission and nuclear fusion.
Nuclei of intermediate size have the highest associated binding energy.
These are therefore more stable than materials having lighter and heavier nucleus.
To create atoms of a stable atom with a high binding energy levels from lighter
atoms (Fusion) or heavier atoms (Fission) results in the release of energy.
Nuclear fission occurs by making the nucleus unstable by causing the nucleus
of a heavy atom to absorb an additional neutron. Following the absorbtion
of the neutron the excited nucleus splits into two almost equal parts. The fission products
include a range of elements of mass number 72 to 160. . The fission reaction also results
in the release of two or three neutrons and a significant quantity of energy.
In the fusion process the fusion of two nuclei to form a single heavier nuclei results in a more stable system with the release of energy. At typical fusion reaction is
D + D -> He3 + n + 3.25 MeV
To achieve fusion the high atomic repulsion forces must be overcome
before the components can be brought sufficiently close to allow the reactions to
Very high temperatures have to be achieved ( as in a Hydrogen Bomb). The fusion process has been confirmed theoretically but it has not yet been possible
to achieve the conditions such that there is a net gain in the energy obtained by the process.
When nuclei are unstable they undergo radioactive decay in more stable nuclei. Five types of decay are identified below with the resultant external effect.
A positron is a positive electron and an alpha particle is the nucleus of a helium atom.
A nucleus subject to radiactive decay always has a definite probability of decay during any
time interval. The half-life of a radioactive isotope is the time required for half
of any initial quantity to decay.
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Last Updated 28/01/2013