 Ionic bonds occur between a positive ion, called a cation, and a negative ion called an anion, because of the electrostatic attraction between the opposite charges. An ionic substance is referred to as a salt. The cation is usually a metallic ion. A common exception though is the polyatomic ion ammonium, which is good to know. The anion is a non-metal and could be a polyatomic ion also, such as sulfate or nitrate. When a metal is reacted with a non-metal, the new substance will be held together with ionic bonds. Let's look at the reaction between sodium and chlorine. It would be worth having a periodic table with you for this. So I'm going to draw these diagrams to represent a sodium atom and a chlorine atom. Sodium is in group one, period three, and it's element number eleven. So it has eleven protons and eleven electrons. Two go in the first level, eight in the next, and that leaves one valence electron in the third level. Chlorine is in group seven of period three and is element number seventeen. So I draw in its electrons and it has seven valence electrons. I'm ignoring the fact that chlorine is actually a covalent molecule for the moment. We just want to look at what happens to the individual atoms. Now recall that it's energetically stable for an atom to have a full outer shell, like a noble gas, and neither of these two do. Sodium has one too many and chlorine has one too few. So if the very electronegative chlorine takes an electron from the less electronegative sodium, they'll both be happy. And what does this do to the overall charge of the atom? Sodium now has one fewer electron than protons, giving it a charge of plus one. And the chlorine, now called chloride, has one more electron than protons, giving it a charge of minus one. Each atom has become an ion with its preferred valence. Notice that the sodium has become smaller, its outer shell is now the second level, and the chloride has become slightly bigger because it has an extra electron. The final thing that happens is that the positive charge of the sodium ion and the negative charge of the chloride ion attract one another and we have an ionic bond. Notice that exactly one electron was lost by the sodium and one was gained by the chlorine, meaning they have the same charge. And that means the ratio of ions in the final compound is one to one. Because it's only the valence electrons that we're interested in, we can redraw those rather cumbersome atom diagrams in shorthand, only showing the valence electrons. So let's do another example with magnesium and fluorine. Magnesium is in group two, so it has two valence electrons. And fluorine is in group seven, so it has seven valence electrons. In order to attain a full outer shell, the magnesium needs to lose two electrons, but the fluorine atom only needs to gain one. What will happen to the second electron from the magnesium? Well, it can be taken up by a second fluorine atom. So the magnesium becomes a magnesium ion with a charge of plus two, and the two fluorine atoms become fluoride ions, each with a charge of minus one. And that's why the overall formula of this salt is MgF2.