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How it works : Binding Energy |
Binding energy is the energy involved in holding together the component parts of a MOLECULE, an ATOM or a NUCLEUS. It is a term used particularly in connection with the nucleus and is the source of both types of nuclear energy, FISSION and FUSION, which have had such an impact on the modern world.
The realization that large amounts of energy are available from the nuclei of the atoms followed from precise measurements of nuclear masses early this century. In instruments called MASS SPECTROSCOPES,the masses can be accurately calculated by measuring the curvature of the paths taken by charged clusters of particles as they travel through electric and magnetic fields. The measurements reveal something which, taken by itself seems impossible. The masses of the nuclei are less than the total arrived at by adding the masses of the protons and neutrons of which they are formed.
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| An impression of the fusion of two hydrogen atoms to make a helium atom. The blue rings represent electrons, which play no part in the action. The hydrogen nuclei-each consisting of one proton, whose weight is shown as its energy equivalent-join with two neutrons tomake the nucleus of the new atom. In joining, they shed some of their mass in the form of energy, amounting to 28 Me V. It is this released energy that powers the hydrogen bomb. |
This enigma can be resolved only through EINSTEIN'S famous formula E =mc2 (energy equals mass multiplied by the speed of light, squared). MASS and ENERGY are interchangeable and the mass which is missing in the sums on the nucleus is related to the energy which holds the nucleus together. This is the binding energy, which was also known initially as the 'mass defect'. Some of the particles' mass has been converted into energy and to separate the original particles, with their higher total mass, energy has to be supplied to the nucleus to break it up.
When the nuclei of all the chemical elements are examined a crucial fact emerges. Most of the elements have nuclei with a binding energy of about 8 MeV or 8 million electron volts per particle-nearly a thousand times the energy of an electron 'fired' by a television picture tube. The lightest and heaviest elements, however, have less binding energy per particle on account of the structure of their nuclei. The amount of mass that is 'missing' when doing the sums on the nucleus is greater for the elements near the centre of the PERIODIC TABLE; if the light or heavy elements can be converted into these central elements some mass can be liberated as free energy.
To take a specific example, a helium nucleus is built up of two PROTONS and two NEUTRONS. The mass of a proton (the nucleus of the normal hydrogen atom) is about 1.7 millionth of a millionth of a millionth of a millionth of a gramme. This is a rather clumsy unit with which to calculate and, using the mass-energy equivalence, the nuclear physicist usually handles the proton mass in units of energy-a proton is 938 MeV, and a neutron is slightly heavier with 939.5 MeV. The helium nucleus is, however, 3727 MeV or 28 MeV less massive than its constituents: 28 MeVs worth of energy can be released in the formation of a single nucleus. Compared with conventional sources, this is a colossal amount of energy.
Conventional energy sources, say the burning of oil, involve atomic binding energies-they are chemical reactions to do with the binding of the electron clouds surrounding the nuclei in atoms and molecules. Here also, the release of. energy is connected with the conversion of mass- the products of the burned oil have very slightly less mass than the oil itself-but nuclear energy sources, which bring the centre of the atom into play, are several million times more powerful. If all the hydrogen nuclei in a glass of water could be combined to create helium nuclei, enough energy would be liberated to drive an ocean liner across the Atlantic.
This process of joining light elements together to form heavier ones is known as fusion, and it is the process that powers the H (hydrogen) bomb, the sun and the stars. There is a great deal of research under way in an attempt to master fusion in the laboratory so that it can be used in power plants.
At the other end of the table of elements, the breaking up of heavy nuclei such as URANIUM is another source of energy.
This process is known as fission. A uranium nucleus usually breaks up into two almost equal parts (which end up, for example, as nuclei of the elements molybdenum and palladium). Because of the different binding energies of heavy and light atoms about 200 MeV is liberated in such a fission, and this powers the NUCLEAR REACTOR and the nuclear bomb.
Reproduced from HOW IT WORKS p285