CHAPTER 1 ELECTRON THEORY

1.1       STRUCTURE OF THE ATOM

It used to be a common thing for people to say ‘We all use electricity, but no one knows what it really is’.  This is no longer true, ever since, at about the turn of the century, Lord Rutherford gave to the world his theory of the structure of the atom, shown in Figure 1.1.

                                                                      Figure 1.1 NEON ATOM

He said that every atom had a nucleus carrying positive electric charges, and around it, circulating in orbit, were one or more electrons each carrying a single negative electric charge.  Moreover the number of negative electrons were such as exactly to neutralise the positive charge on the nucleus.  The attraction between the negative electrons and the positive nucleus keeps the fast-moving electrons in orbit exactly like the gravitational attraction between the planets and the sun.  And, like in the solar system, the electrons (planets), coloured blue in the figure, are much smaller than the nucleus, being about one-thousandth of its size.

The nucleus itself is not a single element but consists of many particles called ‘protons’, each one carrying a single positive electric charge.  The protons are shown coloured red in the figure, so that normally in all matter there are equal numbers of protons in the nucleus and of the orbiting electrons.

Later it was discovered there was a third type of nuclear element - the ‘neutron’. This is a particle, about the same size as a proton, but without any electrical charge, so that it has no effect on the number of orbiting electrons but merely makes the atom heavier.  These neutrons appear white in the nucleus of the figure. 

The chemical behavior of all the elements which constitute matter depends on the number of orbiting electrons. This number varies from 1 in the lightest element (hydrogen) to 92 in the heaviest (uranium).  Until recently it was thought that the list stopped there, but advances in nuclear physics have identified elements with up to 103 electrons in man-made matter; plutonium has 94.

                                                       Figure 1.2 ELEVEN LIGHTEST ELEMENTS

Figure 1.2 shows the first eleven elements of the list.  The lightest, hydrogen, has one proton and one electron.  The next, helium, has two of each, then comes lithium with three, beryllium with four, and so on to the metal sodium (Na) with eleven.  Some of these elements are well known (hydrogen (1), carbon (6), nitrogen (7), oxygen (8)); others are less common.  The figure shows only the protons in the nucleus.  There are in most cases also neutrons, but as they do not affect the orbiting electrons they are not shown.

One peculiar thing should be noted about the electron orbits: they form themselves into rings (or more strictly ‘shells’).  The innermost ring cannot contain more than two electrons.  Any additional ones go into a second ring (like lithium) until that ring is full, It cannot contain more than eight (neon), after which a third ring begins to fill (sodium).  That too cannot contain more than eight, after which a fourth ring begins to fill.  This has a most important effect on the behavior of electricity.

                                           Figure 1.3 ATOMS UNDER ELECTRIC FIELD

When a ring is full, such as with helium (2) or neon (10), the electrons are tightly bound together and are difficult to displace.  In the upper part of Figure 1.3 a neon atom is shown under the influence of a strong electric field.  The negative electrons are attracted towards the positive end, but, though their orbits are distorted, they are not broken up.  Such elements form the ‘inert gases’ such as helium, neon, argon, which will not combine with anything.

In the lower half of the figure is a sodium atom in the same electric field.  Here there is a lone electron in the third ring, and it is quite loosely bound to the nucleus.  Quite a small electric field is sufficient to break it out of its orbit and cause it to seek the positive pole.  Such elements, where electrons can move easily, form many of the ‘metals’; they are used for most electric conductors.

It is easy to see now why metals conduct electrons easily whereas many other elements do not - the latter are called ‘insulators’.

                                                   Figure 1.4 ELECTRON CIRCULATION

Figure 1.4 shows a conductor between two poles of an electric field.  This field is maintained by an electron ‘pump’, or generator, which is driving them round from left to right in the pump and from right to left in the conductor.  If the conductor is a metal, the electric field continually breaks off electrons which pass from one atom to the next and so on back to the pump.  There is a continuous flow of electrons through the conductor and round the loop - we say there is an ‘electric current’ in the circuit.

For reasons going back into history the charge on an electron was found to be negative, so it is always attracted towards the positive pole - that is to say, in Figure 1.4 the electron flow in the conductor is from right to left, and the left-hand pole must be the positive.  But convention has ordained that currents flow from positive to negative, and therefore the conventional electric current is said to be from left to right - that is, against the electron flow.  The original decision was a disaster, but it is too late to change now, and this is a cross we just have to bear.  In these manuals current will always be considered as flowing from positive to negative.