In this module, we're going to look at how we use Lewis symbols to understand covalent bonding. By the end of this module, you should be able to write the Lewis symbol for any element. We're going to use Lewis dot symbols to represent the number of valence electrons in a particular atom. The Lewis dot symbol includes only the valence electrons, because those are the electrons that are involved in bonding. Whether we're having the transfer of electrons to form an ionic bond or the sharing of electrons to form a covalent bond, it's the valence electrons that are involved. The core electrons are those that are inside the valence and lower energy levels, are our core electrons, and they are not going to be involved in the bonding. Only the outermost electrons for our main group, those will be those in the highest n-value, principle quantum shell, those are our valence electrons. We see here for nitrogen, we can look at it's electron configuration, and we see that nitrogen has five valance electrons. And then we can show the symbol, our chemical symbol, with the electrons, with the valance electrons surrounding it. Notice that I don't begin pairing any electrons until each side of my symbol actually has one electron. And this let's us understand a little bit more about how some of our ionic compounds form, so we can see how many bonds an element is likely to make. There is no really a wrong or a right place to put our pair of electrons. We could have put it on any side of this symbol, as long as we have one pair and three individual electrons drawn. The valence electrons of representative or main group elements are in the shell with the highest n-value. Remember that our representative elements are those in the first two columns of the periodic table or in the last six columns of the periodic table. Basically those elements are the ones that don't include the transition metals in the middle block of our periodic table, as well as the lanthanides and actinides. So for our main group elements, we look for the highest n-value. So I look at some examples here for sodium, magnesium, and sulfur. And what I see is that I have valence electrons in the highest n-value for sodium. So it has one valence electron. When I look at magnesium, it has two valence electrons. And then I look at sulfur and see that it has six valence electrons. Now, I can look at the Lewis symbol for each of these, because I now know the number of electrons. Since sodium only had its one valence electron, I show a single electron on the sodium. Again, it doesn't matter which side I draw it on, on the symbol. All we're trying to represent is the number of valence electrons in that atom. For magnesium, we have two valence electrons. Here I show one on the left and one on the right. I could just as ea, easily have drawn one on top and bottom, or one on the top and the right. It doesn't really matter as long as I show them as unpaired or lone electrons on that symbol. When I get to sulfur and I'll have six valence electrons, and so I need to show that I have two pairs and two individual electrons. And again, whatever side they are on is completely irrelevant as far as what kind of structure it can form. They key is that we have two pairs and two lone electrons. And so we can now go back and look at the Lewis symbols for any of our main group elements, and be able to predict how many electrons it will have around it. And this is going to give us information on building structures, and how we're going to know what type of bonding occurs between two particular atoms. So let's look at an example where you need to identify the incorrect Lewis dot symbol. So when we look at our answer here, we notice that oxygen has five valence electrons in this symbol. And when I look at the periodic table, what I see is that the valence shell includes two S2 and two P4. So it has a total of six valence electrons. And therefore, by Lewis symbol, should have a pair of electrons on one side, another pair of electrons and then individual atoms. And it doesn't really matter how I draw those pairs and individual atoms. These are both equivalent to one another in terms of Lewis symbols. [SOUND] In the next module we're going to look at covalent bonds.
