solution and both get satisfied by forming a
molecule again. Even so they don't stay together long before they split
apart and start wandering again. That's what goes on over and over
again, millions of times, when you dissolve a little salt in a glass of
water.

Now we can see what happens when copper sulphate dissolves. The copper
atom has twenty-nine electrons about its nucleus and all except two of
these are nicely grouped for playing their games about the nucleus. Two
of the electrons are rather out of the game, and are unsatisfied. They
play with the electrons of the part of the molecule which is called
"sulphate," that is, the part formed by the sulphur atom and the four
oxygen atoms. These five atoms of the sulphate part stay together very
well and so we treat them as a group.

The sulphate group and the copper atom stay together as long as they are
not in solution but when they are, they act very much like the sodium
and chlorine which I just described. The molecule splits up into two
ions, one positive and one negative. The positive ion is the copper part
except that two of the electrons which really belong to a copper atom
got left behind because the sulphate part wouldn't give them up. The
rest of the molecule is the negative ion.

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 as 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 positi