Each molecule is formed by six atoms, that is by six little groups of
electrons playing about six little nuclei. About each nucleus there is
going on a game but some of the electrons are playing in the game about
their own nucleus and at the same time taking some part in the game
which is going on around one of the other nuclei. That's why the groups
or atoms stay together as a molecule. When the molecules wander out into
the spaces between the water molecules something happens to this
complicated game.

It will be easiest to see what sort of thing happens if we talk about a
molecule of ordinary table salt, for that has only two atoms in it. One
atom is sodium and one is chlorine. The sodium molecule has eleven
electrons playing around its nucleus. Fairly close to the nucleus there
are two electrons. Then farther away there are eight more and these are
having a perfect game. Then still farther away from the nucleus there is
a single lonely electron.

The atom of chlorine has seventeen electrons which play about its
nucleus. Close to the nucleus there are two. A little farther away there
are eight just as there are in the sodium atom. Then still farther away
there are seven.

I am going to draw a picture (Fig. 1) to show what I mean, but you must
remember that these electrons are not all in the same plane as if they
lay on a sheet of paper, but are scattered all around just as they would
be if they were specks on a ball.

[Illustration: Fig. 1]

You see that the sodium atom has one lonely electron which hasn't any
play fellows and that the chlorine atom has seven in its outside circle.
It appears that eight would make a much better game. Suppose that extra
electron in the sodium atom goes over and plays with those in the
chlorine atom so as to make eight in the outside group as I have shown
Fig. 2. That will be all right as long as it doesn't get out of sight of
its own nucleus because you remember that the sodium nucleus is
responsible for eleven electrons. The lonely electron of the sodium atom
needn't be lonely any more if it can persuade its nucleus to stay so
close to the chlorine atom that it can play in the outer circle of the
chlorine atom.

[Illustration: Fig. 2]

The outer circle of the chlorine atom will then have a better game, for
it will have just the eight that makes a perfect game. This can happen
if the chlorine atom will stay close enough to the sodium atom so that
the outermost electron of