Letters of a Radio-Engineer to His Son

by John Mills

The copper ion is a copper atom which has lost two electrons. The sulphate ion is a combination of one sulphur atom, four oxygen atoms and two electrons which it stole from the copper atom. Just as the sodium ion is unsatisfied because in it there is one more proton than there are electrons, so the copper ion is unsatisfied. As a matter of fact it is twice 22as badly unsatisfied. It has two more protons than it has electrons. We say it has twice the “electrical charge” of the sodium ion.

Just like a sodium ion the copper ion will tend to go toward any place where there are extra electrons which it can get to satisfy its own needs. In much the same way the sulphate ion will go toward places where it can give up its two extra electrons. Sometimes, of course, as ions of these two kinds wander about between the water molecules, they meet and satisfy each other by forming a molecule of copper sulphate. But if they do they will split apart later on; that is, they will “dissociate” as we should say.

Now let’s go on with the kind of batteries I used to make as a boy. You can see that in the solution of copper sulphate at the bottom of the jar there was always present a lot of positive copper ions and of negative sulphate ions.

On top of this solution of copper sulphate I poured very carefully a weak solution of sulphuric acid. As I told you, an acid always has hydrogen in its molecules. Sulphuric acid has molecules formed by two hydrogen atoms and one of the groups which we decided to call sulphate. A better name for this acid would be hydrogen sulphate for that would imply that its molecule is the same as one of copper sulphate, except that the place of the copper is taken by two atoms of hydrogen. It takes two atoms of hydrogen because the copper atom has two lonely electrons while a hydrogen atom only has one. It takes two electrons to fill up the game which the 23electrons of the sulphate group are playing. If it can get these from a single atom, all right; but if it has to get one from each of two atoms, it will do it that way.

I remember when I mixed the sulphuric acid with water that I learned to pour the acid into the water and not the other way around. Spatterings of sulphuric acid are not good for hands or clothes. With this solution I filled the jar almost to the top and then hung over the edge a sort of a crow’s foot shape of cast zinc. The zinc reached down into the sulphuric acid solution. There was a binding post on it to which a wire could be connected. This wire and the one which came from the plate of copper at the bottom were the two terminals of the battery. We called the wire from the copper “positive” and the one from the zinc “negative.”

Now we shall see why and how the battery worked. The molecules of sulphuric acid dissociate in solution just as do those of copper sulphate. When sulphuric acid molecules split, the sulphate part goes away with two electrons which don’t belong to it and each of the hydrogen atoms goes away by itself but without its electron. We call each a “hydrogen ion” but you can see that each is a single proton.

In the two solutions are pieces of zinc and copper. Zinc is like all the rest of the metals in one way. Atoms of metals always have lonely electrons for which there doesn’t seem to be room in the game which is going on around their nuclei. Copper as we saw has two lonely electrons in each atom. Zinc 24also has two. Some metals have one and some two and some even more lonely electrons in each atom.

What happens then is this. The sulphate ions wandering around in the weak solution of sulphuric acid come along beside the zinc plate and beckon to its atoms. The sulphate ions had a great deal rather play the game called “zinc sulphate” than the game called “hydrogen sulphate.” So the zinc atoms leave their places to join with the sulphate ions. But wait a minute! The sulphate ions have two extra electrons which they kept from the hydrogen atoms. They don’t need the two lonely electrons which each zinc atom could bring and so the zinc atom leaves behind it these unnecessary electrons.

Every time a zinc atom leaves the plate it fails to take all its electrons with it. What leaves the zinc plate, therefore, to go into solution is really not a zinc atom but is a zinc ion; that is, it is the nucleus of a zinc atom and all except two of the planetary electrons.

Every time a zinc ion leaves the plate there are left behind two electrons. The plate doesn’t want them for all the rest of its atoms have just the same number of protons as of electrons. Where are they to go? We shall see in a minute.