the sodium atom can play in the chlorine
circle. You see everything will be satisfactory if an electron can be
shared by the two atoms. That can happen only if the two atoms stay
together; that is, if they form a molecule. That's why there are
molecules and that's what I meant when I spoke of the molecule as a big
game played by the electrons of two or more atoms.

This molecule which is formed by a sodium atom and a chlorine atom is
called a molecule of sodium chloride by chemists and a molecule of salt
by most every one who eats it. Something strange happens when it
dissolves. It wanders around between the water molecules and for some
reason or other--we don't know exactly why--it decides to split up again
into sodium and chlorine but it can't quite do it. The electron which
joined the game about the chlorine nucleus won't leave it. The result is
that the nucleus of the sodium atom gets away but it leaves this one
electron behind.

What gets away isn't a sodium atom for it has one too few electrons; and
what remains behind isn't a chlorine atom for it has one too many
electrons. We call these new groups "ions" from a Greek word which means
"to go" for they do go, wandering off into the spaces between the water
molecules. Fig. 3 gives you an idea of what happens.

You remember that in an atom there are always just as many protons as
electrons. In this sodium ion which is formed when the nucleus of the
sodium atom breaks away but leaves behind one planetary electron, there
is then one more proton than there are electrons. Because it has an
extra proton, which hasn't any electron to associate with, we call it a
plus ion or a "positive ion." Similarly we call the chlorine ion, which
has one less proton than it has electrons, a minus or "negative ion."

[Illustration: Fig. 3]

Now, despite the fact that these ions broke away from each other they
aren't really satisfied. Any time that the sodium ion can find an
electron to take the place of the one it lost it will welcome it. That
is, the sodium ion will want to go toward places where there are extra
electrons. In the same way the chlorine ion will go toward places where
electrons are wanted as if it could satisfy its guilty conscience by
giving up the electron which it stole from the sodium atom, or at least
by giving away some other electron, for they are all alike anyway.

Sometimes a positive sodium ion and a negative chlorine ion meet in
their wanderings in the