Fig. 3.I. Shells in which electrons rotate.

An atom is small. 250,000,000 atoms laid side by side in a row extend one inch. Yet the central nucleus of the inconceivably small atom occupies only one ten-thousandth of the diameter of the atom, and an electron is only about one-fifth the diameter of the nucleus. The only parts of the nucleus that interest us now are called protons, which are particles of positive electricity. We are much more interested in the electrons, which are particles of negative electricity. Our greater interest in the electrons arises from the fact that they can be separated from their atoms, and can be moved about and controlled. Electrons are the electricity with which we do things.

Positive and negative electricity, that is, protons and electrons have a strong attraction for each other. This attraction between the positive protons and the negative electrons holds the atom together.

ATOMS AND ELECTRONS

Now let's talk about a familiar substance — aluminium. In each atom of aluminium there are 13 negative electrons whirling around the positive nucleus. The attraction between these 13 particles of negative electri-city and the positive protons in the nucleus is sufficient to hold the negative electrons in the atom under normal conditions. The positive force in the nucleus is equal to the negative forces of the 13 electrons as has been indicated in fig.3.2.

In the atoms of aluminium and of many other substances one of the negative electrons farthest from the nucleus frequently breaks away from the atom to become a free electron. Ordinarily these free electrons almost immediately enter other atoms which have lost an electron because of the attraction between the positive electricity in the atoms and the negative electricity of the free electrons.

Fig. 3.2. The positive forces in the nucleus, represented by plus (+) signs, exactly balance the negative forces of all the electrons, represented by minus. (—) signs.

Fig. 3.3. Positive Atom. The aluminium atom has lost one electron from its outer shell, leaving the atom positive while the electron becomes a free particle of negative electricity.

Assume that an aluminium atom has lost one of its negative electrons, as in fig.3.3. That free electron now is negative, it is a wandering particle of negative electricity. The nucleus of the atom still has as much positive electricity as ever, it still has enough positive electricity to attract and hold 13 negative electrons, but actually there are only 12 electrons remaining in the atom.

This atom which has lost an electron contains more positive electricity than negative electricity. So we may call it a positive atom. This positive atom has strong attraction for any free negative electrons in its vicinity and that is the reason that remaining free electrons are continually re-entering the atoms.

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