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 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.