 Alright for this little part of the video it will help if you have a periodic table in front of you so pause the video now and go and grab one. Remember in the last video we found that we could read the number of valence electrons in atoms by looking at which vertical group the element was in on the periodic table. We can use that same information to work out what kind of ion the element most likes to form. We do this using a rule of thumb called the octet rule. This says that it's energetically favorable for an atom to lose or gain the number of electrons that will give it a full outer shell, so a full valence shell. For instance sodium has one valence electron, it's in group one. To get a full outer shell it can either gain seven electrons to make its outer shell up to eight or it could lose one. Its outer shell would now be the next level down but that's okay it would have a full eight electrons. It's less complicated for the atom to lose one than it is to gain seven so it loses one and if it loses one electron it means it now has one more proton than electron which means it has a charge of plus one. So the ion that sodium forms is a plus one charged ion and we write it Na plus. Now look at magnesium it's in group two so it has two valence electrons that means it can either gain six to get a full eight or it can lose two. Losing two is less complicated so it does that. That means it now has two more protons than electrons and that gives it a charge of plus two and we write that as mg2+. What about fluorine? It's in group seven so it has seven valence electrons. It could either gain one or lose seven. Again we go for the least complicated option which in this case is to gain one electron. This means that the fluoride ion has one more electron than it has protons and that gives it a charge of minus one. So we write the fluoride ion as f-. One final example. Lithium is element number three. It has three electrons and three protons. It can either lose one electron or gain seven to get a full outer shell. Losing one is the easiest option so it does that. But that leaves it with only two electrons and this is called the octet rule. Well recall that the innermost electron level can hold only two electrons. We call this the octet rule because it's an easy name and the second and third levels are very stable with eight electrons each. But really it should be called something like the full electron level rule. The fact that the outer shell of a lithium ion has two rather than eight electrons is okay because that shell can only hold a maximum of two. So the lithium ion has a charge of plus one and we write it Li plus.