THE SECRET OF THE ATOM
By PROFESSOR HERBER'F DINGLE LL matter is composed of atoms—tiny systems of particles each maintaining a separate existence in every portion of solid, liquid or gas that exists. Each atom is made up, according to our present knowledge, of at least three kinds of particles—electrons, protons, and neutrons—and according to their arrangement in the atom we have one kind of element or another.
The general structure of an atom is as follows: At the centre there is a "nucleus" containing protons and neutrons only, and travelling round it, like planets round the sun, are the electrons. These electrons are the lightest of the three particles ; a pound of them would contain as many as 5 followed by 29 noughts. Let us, for simplicity, call the weight of a single electron one unit. Then the proton and neutron weigh each about 1,836 units, so that the nucleus of the atom contains almost the whole of its weight. The proton has a positive electric charge, and the electron an equal negative electric charge ; the neutron, as its name implies, has no charge at all. Hence the nucleus of the atom has, on the whole, a positive charge, and the surrounding electrons, which are always equal in number to the nuclear protons, have all together an equal negative charge. The atom, as a whole, is therefore electrically neutral.
One element differs from another simply in the number of these protons or electrons. The lightest atom, hydrogen, has one proton in the nucleus, and one electron revolving round it. The next element, helium, has two nuclear protons and two revolving elec- trons; lithium has three, beryllium four, and so on up to uranium with ninety-two of each. There are one or two gaps in this series, corresponding to possible elements not yet discovered. The number of an element in the series—i.e., the number of protons in its nucleus or electrons revolving round it—is called its "atomic number."
The number of neutrons in the nucleus is not always the same, even for a single element, but it is usually not very different from the number of protons. The addition or subtraction of a neutron makes no difference to the positive charge in the nucleus, and there- fore no difference to the number of negative electrons required to balance that charge, but it does affect the weight of the atom. The chemical properties of an element are determined almost entirely by the revolving electrons, so that atom's chemically identical can have various weights. Hydrogen, for example, has two kinds of
atoms. In one the nucleus contains a single proton, and in the other a proton and a neutron. (This latter is a constituent of "heavy water.") Each atom, therefore, requires only one external elec- tron, and the chemical properties of the two are similar, but the second atom is twice as massive as the first. These different forms of the same element are called isotopes. They are chemically in- distinguishable. but physically different.
The atoms of most elements are stable structures, but some of the heaviest ones are known to break up spontmeously, emitting particles and etherial waves. Uranium and radium are the best known of these, and the phenomenon is called "radio-activity." In a typical case a compact group of two protons and two neutrons, called an alpha particle (which is, in fact, the same as the nucleus of an atom of helium) is sent out from the nucleus of a radio-active atom. ThiE reduces the nuclear charge by two units. Two re- volving electrons therefore become superfluous, and electrons also (known as beta particles) are sent out with the alpha-particles. The atom of the radio-active element is thus reduced in atomic number by 2 and so becomes an atom of a different element. The re- arrangement of the particles which are left into the normal con- figuration for the new element is then accompanied by the emission of very hard X-rays (etherial waves) known as gamma rays.
These naturally-occurring changes throw great light on the struc- ture of the atom, but for our present purpose their chief importance is that the particles emitted (which come out with very great speeds, approaching that of light) can be used as projectiles with which to bombard other atoms. The pioneer in this process was the late Lord Rutherford. The purpose of doing this is to discover more about their structure, and it is in this way, in tact, that the neutron was knocked out of the atomic nuclei and its existence discovered by Sir James Chadwick. This bombardment of atoms has for some time been our chief means of learning the secrets of the atomic nucleus.
In any configuration of these ultimate particles there is a great deal of what is called potential energy, of which we know very little except that in suitable circumstances it can be converted into kinetic energy—i.e., the energy of motion of matter. If we can rearrange the particles in an atom, we may release some of this energy or, on the other hand, we may have to supply energy in order to rearrange it. Both things happen, and it is a matter of experiment to determine what occurs in any particular case. The earliest known examples of this are concerned with the rearrange- ment of the outer electrons in atoms, and the simplest way of making such a rearrangeme at is to bring two atoms of different substances together and to see what happens.
Up to the present time all explosives (chemical explosives they may be called) have been of this character—configurations of atoms of different elements which need only a slight stimulus to cause them to rearrange themselves into different groupings with, the release of energy. In all sucn changes the nuclei of the atoms remain unchanged ; it is the energy of the electrons alone that is released. There is reason to believe, however, that the potential energy in the nucleus itself is enormously greater than that of the electrons. We know very little of the forces which hold the nucleus together, but we do know that they must be extremely great, for, if the ordinary laws of electrical repulsion operated in so minute a space, the protons, having no negative charges to keep them in place, would repel one another with such energy that the whole universe would be shattered.
Up to the time when research on these subjects became secret the most direct way of discovering how to release nuclear energy was by experiment. Various atoms were bombarded by various particles and the results were observed. Sometimes the particles stuck to the atoms hit, and changed them into heavier ones. Some- times the reverse occurred, and atoms were broken down into lighter ones. In such experiments comparatively few atoms were changed, because, owing to the smallness of the nuclei, a hit was a rare occurrence ; aiming ts impossible—you fire and hope for the best. We can calculate, however, the potential energy released per atom in the following way.
Potential energy has weight, and the weight of an atomic nucleus is not only that of the constituent particles but that of the potential energy of their configuration also. If then an atom is broken into two, and the sum of their weights is not equal to the weight of the original atom, the balance is the weight of potential energy, which reappears probably as kinetic energy of the atoms produced. The energy from one or two atoms may do little harm, but if a way could be found of disintegrating even one in a million or less of a tiny fragment of substance, the effects might far transcend anything pre- viously obtained. So far as can be judged from the information available this would appear now to have been done.
Though we cannot say, apart from experiment, how much energy can be released from atomic nuclei, we can set a maximum to it, but this is so enormous that it still leaves practical possibilities almost infinite. The amount of energy theoretically obtainable from an atom is obtained by multiplying its mass by the square of the velocity of light. This means that a mass of one gram might conceivably yield 900 million million million ergs, or units of energy. This would be enough to raise 200,000 tons of water from the freezing to the boiling-point, and it takes 453 grams to equal a pound.
The possibility of controlling this vast store of energy depends on the discovery of a means of limiting the number of atomic nuclei broken up. The energy obtained from a particular single disintegra- tion, or " fission " is fixed. It may be that a means of control is already known. In the break-up of some nuclei it is known that a series of changes is started which it would be very difficult, if not impossible, to control, but in other cases the process might be more tractable. Certainly the probabilities are that further research will show how we can harnecs sub-atomic energy, even if that is not already possible.
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