Learn about Junior Chemistry, Chemical Bonding 3, in this comprehensive video by bannanaiscool.
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Rob Lederer: Okay, this is not the maximal way that methane CH4 can actually be drawn. The C can be attached H is not a 90 degree fashion that you see here in two dimensions, but in three dimension like this. Look at that, that's called a tetrahedral shape. So tetrahedral because, there is four bonds or four effective pairs. Four effective pairs got tetrahedral arrangement of electrons. Now, in this tetrahedral that you see here, a tetrahedral shape, you have got 109 degree bond angles between these elements, a lot better than 90 between the hydrogens here and so this maximizes in three dimensions from the way the molecule would look, tetrahedral. But, if we have something like ammonia which also has one, two, three, four effective pairs but one lone pair and three bonding pairs, that's going to have a bit of a different shape. So, you just take this. suck the top half of that and put it in pair of electrons appear, but something called valence shell electron pair repulsion theory tells us, that the lone pairs occupy more space than bonding electrons. So, they force the lone pair up here, forces the bonds down here to about a 107 degree difference. And ammonia looks like that, we just put the nitrogen in there, blue instead of black, okay fine. So there is your pyramidal shape for this. Now, in the tetrahedral, we look at that and we say, you know all those bonding was look equivalent to me and because they are all way around, that is a non-polar molecule. But, in this one nitrogens were more electro negative than hydrogen, so that line pointing from the hydrogen to nitrate going this way, going this way, going this way, none of them cancel out without anything coming down like you did in the tetrahedral. Pyramidal shapes give you polar. Now how about water, which is bent. Well, just taking the pyridamal I going, and now you have got a V shape or a bent shape and again, it's is a polar molecule because this line and this line don't cancel out and water is bent shaped. Where are those lone pairs again? Up here and over here forming a tetrahedral. So, its tetrahedral structure but, its got two lone pairs. We don't cancel long pairs in the name of the shape and that's a V shape or bent. Okay so, then how about HF? well that's pretty obvious isn't it. You are going to have to have the a linear shape for that. That's called linear. Effective pair is here, here and here. They are in a tetrahedral arrangement but that's a linear molecule. A three effective pairs is a little different. Just they look like three effective pairs, but doesn't for this formula C2H4, when you build it like this, it looks like you got one, two, three, four effective pairs around that carbon. So, it should be tetrahedral. Now count multiple bonds as one effective pair. That's the rule. that's the bending of the rule you have to do here. Multiple bond is what this carbon has one, two, three effective pairs and when you add that, the maximal distance that the electrons can get away in terms of the bond angles in a circle is 120 degrees. So, in this molecule here, you have a carbon attached to a carbon, this carbon is also attached to a hydrogen in here and here. If you look at this much of it right there, there and there. They are all a 120 degrees away from each other, flat in a plane, trigonal planer. They were carbon dioxide from before, had double bonds going to the oxygens at either end. There you go, those are springs that are in there. Sure now, this carbon has how many effective pairs? One, two. Count the multiple bonds as one. Two effective pairs always give you a linear shape.