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Video transcript

the Lewis electron dot diagram for c2h4 is shown below in the box on the left in the Box on the right complete the Lewis electron dot diagram for c2h5oh or ethanol by drawing in all of the electron pairs so as I said this right over here this is the Lewis electron dot diagram for ethene and they want us to fill in all of the by draw they wants to draw on all the electron pairs for ethanol and what we could do and I'll do it in a way where we can see which electron comes from which atom but they're not even asking you to do that but let in blue I'm going to make the electrons from the hydrogen's so each hydrogen you can think of it as contributing one electron to each pair and that's what's forming the covalent bonds so say this hydrogen is going to contribute this electron this hydrogen can contribute this electron this hydrogen can contribute this electron this hydrogen can contribute that electron this hydrogen can contribute that electron this hydrogen can contribute this this electron right over there and then let's see so now let's think about the carbons and if you're actually taking the AP test you wouldn't probably don't even have markers around but I'm gonna do it in different colors as you can see it so each carbon has four valence electrons that it can contribute for these before covalent bonding so this carbon right over here that's one two three and then four and now this carbon over here can contribute one two three and four and now we think about the oxygen so oxygen is interesting it's going to have two lone pairs so it's going to have one lone pair I could do there one lone pair like that and then it's going to have is going to form this bond by contributing one electron to this pair and one electron to this pair right over there so each of these pairs represent a covalent bond this one I could have drawn a little bit lower but I think you get the idea those I have drawn in all of the electron pairs now let's think about Part II what is the approximate value of the carbon oxygen hydrogen bond angle in the ethanol molecule so in the ethanol they're really saying so the ethanol molecule they want to know the bond angle you have the oxygen bonded to a hydrogen bonded to the c2h5 so I'll just write that is c2 I'm sorry c2h5 like that and they want to know what is approximately this bond angle going to be and the important thing to realize is when you form these bonds with oxygen it's going to have pretty close to a tetrahedral shape why because the oxygen has these two lone pairs so these two lone pairs are forming the other parts the other the other I guess you could say points of the the tetrahedral shape and so if we were talking about just water so water if we're talking about a water molecule right over here and I'm not doing a good job of drying it you could draw one electron pair there and then the other electron pair would be here in the back actually I could let me draw it like this let me draw I could draw it like this I could if we were talking about water I could draw one hydrogen popping out I could draw the other hydrogen popping in and then one electron pair is over here and one electron pair is over here this bond angle over here in water and this is I guess a semi useful thing to know in general you could try to eyeball and say oh that looks a little a little bit more than 100 degrees and you'd be right this is approximately 100 point four point five degrees if you and this is actually not a perfect tetrahedral shape it gets distorted because these lone pairs of electrons are repelling each other and making these two getting a little bit closer together with each other if you have a pure tetrahedral shape so you have something in the middle well let me just if you had a pure tetrahedral shape like this so let me so let me draw it like this so something popping out you have something something going in and then you have two things like this that's one way to think about a tetrahedron there's others then the bond angle between all of these if it's a more I guess you could say symmetric tetra non a non distorted tetrahedral shape is 109.5 degrees and this is a reasonable a reasonably useful number to know obviously for this question as well I don't know if you can if this is obvious let me actually draw the tetra he'd let me let me connect the tetrahedron so you could draw it like that and then that would be the other side right there I don't know if that helps let me draw another one so if I were to draw a tetrahedron and if this was transparent you have a molecule or you have an atom in the middle and then you have the four bonds 1 2 3 4 I could say 4 bonds or lone pairs then let me make sure you can see the one in the center then the angle here is approximately 109 point five degrees so this is going to be tetrahedral but you have these lone pairs that are going to be repelling each other a little bit so you're going to be someplace in the neighborhood of I don't know around where water is or a more pure tetrahedral shape so I would say your bond angle is going to be I don't know between 104 and 110 degrees 104 to 110 degrees in fact I would I would estimate that it's going to be more than hundred four point five because these the well I'll just I won't I won't try to dig too much into it they really just want us to approximate approximate the value so you could give a you know anything in this range would be would be a suitable suitable answer
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