1.1 Structure of the atom

For our purpose of understanding a nuclear reactor it is sufficient to regard the atom as consisting of a very dense small nucleus, made up of proton and neutron particles, around which move particulate electrons in well defined orbits. This model of the atom has been likened to a planetary system with the sun as the central nucleus and the planets as the orbiting electrons — see Fig 1.1 illustrating the atom of lithium. The structure of the atom is now known to be much more complex than this but the model, despite its simplicity, has enabled the successful development of nuclear power generation.

The masses of a proton and a neutron are similar and much heavier than an electron, by a factor of 1840. As the nucleus contains ail the protons and neutrons, known collectively as nucleons, it follows that the mass of the atom is predominantly concen­trated in the nucleus.

The proton carries a unit of positive electrical charge (1,6022 x 10“ 19 coulomb) and the neutron is electrically neutral. Each electron carries a unit of negative charge equal in magnitude to the positive charge on a proton. The number of orbital electrons is equal to the number of protons in the nucleus so that their charges balance and overall the atom is electrically neutral. (Should an atom lose or gain one or more planetary electrons it is then left with a residua] electric charge and the atom is said to be ionised.) Thus the number of protons in the nucleus determines the number of orbiting electrons. It is the

Fig. 1.1 Structure of the atom

electrons, and in particular the outermost orbiting electrons, that give the atom its chemical properties. Chemical reactions are due to electron interactions.

The atom is extremely small — a drop of water contains several thousand million million million atoms. Even so, an atom consists mainly of empty space. Atomic diameters, the diameter of the electron orbits, are of the order of 10~’° m whereas the diameter of the nucleus is of the order of 10"14 m, ten thousand times smaller. If Fig 1.1 was shown to scale, and taking the scaled diameter of the nucleus in the figure to be of the order of 1 cm, the electrons would need to be shown orbiting the nucleus at a distance of 100 m or so.