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Studying for a test? Prepare with these 4 lessons on Structure and bonding.
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Video transcript
In this video, we're going to look at how to draw dot structures of simple organic molecules that have single bonds. So if I look at the molecular formula CH4, which is methane, and I want to draw a dot structure for the methane molecule, I would go over here to my organic periodic table and find carbon. And I can see carbon is in group IV. Therefore, carbon will have four valence electrons. So I can draw a carbon with its four valence electrons around it like that. Remember from general chemistry, valence electrons are the electrons in the outermost energy level. So carbon has four valence electrons in its outermost energy level. Next, I have to think about hydrogen. And hydrogen is in group I on the periodic table. Therefore, hydrogen will have one valence electron. And so I can go ahead and put a hydrogen in there with one valence electron. And I know I have to do that three more times. So I keep putting in hydrogens, each with one valence electron, so a total of four hydrogens. And now I can start connecting my dots. I know that two valence electrons equals one single covalent bond. So there is a single covalent bond. There is a single covalent bond. And then I have two more here. So this is my complete dot structure for methane. Now I can see that carbon is surrounded by eight electrons here. So we can go ahead and highlight those. So if I'm counting the electrons around carbon, it would be two, four, six, and eight, like that. And eight electrons around carbon makes carbon very stable. And if we look at the periodic table, we can see why. So if I look at the second period, I can see that the valence electrons for carbon would be one, two, three, and four. And to get to eight electrons, we would go five, six, seven, eight. So if carbon is surrounded by eight electrons, it's like it has the electron configuration of a noble gas, which makes it very stable, because all of the orbitals in that energy level are now full. So an octet of electrons is the maximum number of electrons for carbon. If we look at hydrogen, we can see that each hydrogen is surrounded by two electrons. And so if I find hydrogen here, hydrogen is in the first energy level. And so here's one electron and here's two electrons. So in the first energy level, there is only an s orbital. And so that s orbital holds a maximum of two electrons. And we get to the electron configuration of a noble gas. And so hydrogen is stable with having only two electrons around it. Let's look at another dot structure. And let's do one that has nitrogen in it. So if I look at the molecular formula CH3 NH2, I'm going to once again start with carbon in the center with its four valence electrons around it, like that. And I know that there are three hydrogens on that carbon. So I can go ahead and put in those three hydrogens. Each hydrogen has one valence electron, like that. And then on the right side, I'm going to think about nitrogen. So I need to find nitrogen on my periodic table. Nitrogen is in group V. Therefore, nitrogen has five valence electrons. I can represent those valence electrons as one, two, three, four, and five, like that. And I still have two hydrogens to worry about, right? So I have still have these two hydrogens here. And I can see there's a place for them on the nitrogen. So I can go ahead and put a hydrogen in here and a hydrogen in here, and connect the dots. And I have my dot structure. And I can also check on my octet rule. So carbon has an octet. And nitrogen has an octet as well. So let's go ahead and verify that. So there's two electrons here, four, six, and eight. So nitrogen is in the second period. And so nitrogen is also going to follow the octet rule when you're drawing your dot structures. Let's do one with oxygen next. So if I wanted to draw the dot structure for methanol, methanol is CH3 OH. And so once again, I start with carbon with the four valence electrons. And I have three hydrogens, each one with one valence electron, like that. And so I can go ahead and put in those three hydrogens. Next I have oxygen. So I need to find oxygen on my organic periodic table. And I can see that oxygen is in group VI right here. So oxygen is going to have six valence electrons around it. So I can go ahead and draw in oxygen. And I can put its six valence electrons in-- one, two, three, four, five, and six, like that. And then I'm going to put in the hydrogen, right? So now I have a hydrogen to worry about. And I know that hydrogen has one valence electron. So I can see there's a place for it over here. And once again, I can connect the dots and see all of the single covalent bonds in this molecule. So that's one bond. That's another bond. And then I can see the carbon has bonded to the oxygen. And the oxygen has bonded to this hydrogen as well. Again, we can check our octet rule. So the carbon has eight electrons around it. And so does the oxygen. So this would be two right here, and then four, and then six, and then eight. So oxygen is going to follow the octet rule. Now when you're drawing dot structures, you don't always have to do this step where you're drawing each individual atom and summing all of your valence electrons that way. You can just start drawing it. So for an example, if I gave you C2 H6, which is ethane, another way to do it would just be starting to draw some bonds here. And so I have two carbons. And it's a pretty good bet those two carbons are going to be connected to each other. And then I have six hydrogens. And if I look at what's possible around those carbons, I could put those six hydrogens around those two carbon atoms, like that. And if I do that, I'll have an octet around each carbon atom. So this would be my dot structure for ethane. To double check yourself, you could make sure that your dot structure has the correct number of valence electrons. So if I'm thinking about each carbon having four valence electrons and I have two of them, I'm going to get eight valence electrons from those two carbons that I have to represent in my dot structure. Each hydrogen has one valence electron. And I have six of them, so I need to worry about six valence electrons from the hydrogen, so for a total of 14. So when I look at my dot structure, I can check to make sure I have the correct number of valence electrons. I need 14. So let's go ahead and count them. So this would be two here, four, six, eight, 10, 12 and 14. So I have the correct number of valence electrons represented in my dot structure. I also have an octet of electrons around my carbons. And so this would be the dot structure for ethane.