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Polar and Non Polar Covalent Molecules, Polar vs. Nonpolar - CLEAR & SIMPLE

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Hey dudes, it's Mr. Post and on today's video we'll be checking out a very complicated topic known as molecular polarity. And the goal for you today is simply to leave this lesson saying can I look at any molecule and classify it as either polar or nonpolar. Molecules will all fall within this range of being polar or nonpolar, and there's actually varying degrees to which you are polar. The 'gist' of today's video is not really the varying degrees in which one is more polar than another but rather simply can you look just at a molecule and say 'Is it polar or nonpolar?' simply based upon its shape. So molecular polarity does depend upon two things: the symmetry of the shape of a molecule, and also the polarity of the bond itself. Now just so we're crystal clear, this here is a bond, that's a bond, and that's a bond. There are 3 covalent bonds in that molecule I've identified. The whole entire molecule though is what we're talking about today. This is molecular polarity not bond polarity. So we're going to be focused on the whole entire molecule and the symmetry of that shape. Nonpolar molecules are molecules that do not have positive and negative sides. Like a good example here like this the whole entire molecule - it's a trigonal planar and let's just say it's like Bf3 - boron trifluoride. You know one thing you might see with boron trifluoride is that all these fluorines are actually negative. And although they're actually negative and the middle might be positive I do not have a positive and negative side; I don't have a dipole. When the middle's positive what I end up with here is a negative/negative end and that is not a pole. A pole has to be where one side is totally positive and one side is totally negative. So nonpolar molecules tend to be symmetrical. Alright, this trigonal planar shape is symmetrical; it's the same on all sides. On this side it's a white molecule bond to a white molecule this side a white molecule bond with white molecule and likewise this side this side a white molecule bond with a white molecule. for the sake., of you know, explaining this. Polar molecules though they do have a pole, a plus and minus side. One side the whole molecule is positive and one side the whole molecule is negative and and we tend to say their shape is asymmetrical. It is not the same all over. The one example of an asymmetrical molecule here is the bent molecule. Bent molecules are asymmetrical You'll notice at the bottom here is different than the top and because of that it gives it a polar nature where this side could be perhaps positive and this side can be perhaps negative; it doesn't have to be that way though. The causes of polarity are 1. the asymmetrical shape I want you to know that whenever you see a bent molecule it is polar. Anytime you to lewis dot structure and you do a throw down to the shape and it's bent it is a polar molecule. anytime you see trigonal pyramidal it is also a polar molecule too. Once you notice the characteristics of these molecules that they have unshared electrons on their central atom and that causes the valence of electrons to bend away These are characteristics of polar molecules. also polar bonds can contribute to a molecule being polar; it does not always have to have polar bonds but it can. One of the things I want to get across is that just because you have a shape such as tetrahedral a tetrahedral can lend itself to be a symmetrical shape in this case it is asymmetrical because this other element over here is different than the other three. So I could have asymmetrical due to a shape of bent or pyramidal and also asymmetrical when I do have a symmetrical shape but the outside is different somewhere. And we'll go over examples of all these. So what I want you to leave here with now is knowing that anytime I have a tetrahedral, a linear, and a trigonal planar shape, they can either be polar or nonpolar. And that depends upon the symmetry within that molecule. Are all the end of the molecule the same or is one of them different? Bent or pyramidal are always. always polar and they always have it asymmetrical shape that will lend to that polarity. Some examples of symmetrical and asymmetrical: this is a symmetrical tetrahedral shape which is would be considered a nonpolar molecule and this is an asymmetrical tetrahedral shape because I have asymmetry here it is not the same on all sides it is considered a polar molecule. Here we have trigonal planar and here we have trigonal pyramidal: and I want to say the sides are these, these are the sides alright the sides are white connected to white connected to white connected to white connected to white and this is the middle, the middle is not a side unless the middle is elevated and that's what we have here in trigonal pyramidal the middle is actually an elevated structure here where I do have sides of my molecule if you were to build it like this molecular model kit is. So symmetrical leads to nonpolar molecules asymmetrical to a varying degrees it will always polar. Once again, what's the main difference here? The trigonal planar lacks an extra pair of electrons because it's a group 3 element. Trigonal pyramidal will have an extra pair of electrons on the central atom which will cause other electrons to bend downward and this be elevated upwards. Now the middle does have a polarity that is different than the other sides. How about this shape here guys? We have something like CO2 and something like H2O. Once again: this is linear, it is the same on the sides, these sides will contribute to a negative perhaps charge and have a negative/negative end that is not a pole. This here though gives me a one side of the molecule that can perhaps positive and the other side negative, and that would lead to a symmetry and a pole being created. A pole where one side is positive and one side is negative. What's the main difference between these two molecules? Well they both have three atoms but this one's got the extra pair of electrons that carbon dioxide does not have on the central atom. Carbon dioxide's central atom lacks that therefore it leads to a symmetrical shape going across and linear. Okay guys here's where it comes down to; I want you to start trying these examples. Are these examples of polar or nonpolar molecules? Here we're looking at any one of these. it can be carbon tetrafluoride could be methane, silicon tetrafluoride. The thing is they're both group four elements. They'll be looking like this in the Lewis Dot structure, and all the Lewis Dot structures have this shape. Is this a symmetrical shape or is it an asymmetrical shape? That's the first thing you have to ask yourself. Alright, take two seconds guys, answer the questions from now on. Take a shot at it. Press pause and come back with the answer. This is a nonpolar molecule it's symmetrical all over the place; every single outside is identical to the other other outside. Therefore it is symmetrical and nonpolar. BF3 - boron trifluoride. The bonds on these are very, very polar but is the molecule polar? In this case the molecule is nonpolar because it is a symmetrical shape no matter how you spin this it's always going to look the same. Let's check this out guys. Either of the compounds here NF3 or PF3, whatever one you want to use, they both have the same Lewis Dot structure because they're both group five or 15 elements. I want you to look at this down here: is this a symmetrical shape or asymmetrical shape classified as polar or nonpolar? This is a polar molecule. Trigonal pyramidal is always going to be a polar molecule. In this case I have differences: these three are going to be different than the top one here. Therefore that's going to lend me to have a polar molecule. What's going on with H2O or H2S? Well either one you choose is going to be a polar molecule. I've already said that bent structure is a polar molecule so therefore you already know coming in, when you see bent it is polar. C02. What's going on? Well the form is in the shape of linear. Linear is an example where it can either be polar or nonpolar. But, because of my ends, the "O"'s are idential in this case silicon dioxide also, if the ends are identical therefore the molecule is going to be nonpolar. There is no pole present here. Nitrogen is a great example of a linear molecule that is the same on both sides. When you're same on both sides you're going to be nonpolar. Carbon tetrafluoride: this is a tetrahedral sja[e and you'll notice that exterior is all the same. Each one of these is a fluorine: that's a fluorine, that's a fluorine, so really if fluorine is going to take on a negative charge due to its electronegativity, which it will, I want you to notice this. You have negatives all over the place on the outside, and yeah the middle is + but the side's kind of what matters. These sides are all going to be negative sides; negative charges. Therefore although you have very polar bonds, you have a nonpolar molecule. The example here is Fluoromethane or Methyl Fluoride. Take a look at it guys. Please decide if it is polar or nonpolar. Well if these three atoms on the sides are hydrogen and this one is a fluorine clearly this asymmetrical. This is not the same throughout anymore I have three H's and one 'F' now this is definitely gonig to be a polar molecule. How about oxygen? Well oxygen should be pretty easy guys. It's linear and it can go either way; it can either be a nonpolar or it can be polar. The fact that I have the same atom on both sides though kind of lends me to thinking, yes, this is going to be a nonpolar molecule. What about hydrogen monofluoride, also known as hydrogen fluoride? How do you take this one? It is linear. Is it symmetrical or is it asymmetrical? Well checking this out guys, you'll notice that I split the atom in half - the molecule in half - I have one side that's fluroine and one side that's hydrogen they're not the same. They're not symmetrical. This would be considered an actually very, very polar molecule. What do you think about oxygen difluoride? Oxygen difluoride happens to be a polar molecule. It's a polar molecule, primarily, because of it's bent shape. You have two unshared pairs of electrons up here that are going to cause the structure to be repelled downwards and as it is repelled downwards now I have a bottom half that is different than the top half. And that causes a polar molecule. So here we have polar. This one shouldn't be too hard. I think every one you've seen so far that's been N2 , O2 this is F2, fluorine is the name of the molecule, it is the same on both sides it is almost like a mirror image here guys thats total symmetry. So symmetrical molecules are nonpolar. Check out the picture here this is a molecular model of ethanol. And this is the Lewis Dot structure down here. So if you look at the Lewis Dot structure you see I have this O-H ending. O-H endings always indicate alchohol group. An alcohol group was attached to ethane and in this case if we key on this over here, just looking at that, do you think this picture shows a symmetrical molecule? So what I want to really demonstrate its just by looking at the symmetry of the molecule you can predict it's polarity. You're not going to be able to tell how polar it is, but you will be able to tell that it is polar. This example of clearly a polar molecule. That one side is clearly different than other side so ehtanol is a polar molecule. This is a Lewis Dot structure of cycropentane. If you look at the picture here it kind of makes it a little easier to see perhaps, but it's the same. Every carbon is attached to two hydrogenss and also attached two carbons. No matter which way you look at it, it's identical; you know this one down here is the same as this one over here and because it has that symmetry cycropentane is nonpolar. But on the next slide I'm going to make it polar. I added the alcohol group the "O-H" to one of the carbons, and in doing so I have altered the symmetry. Now this is asymmetrical and this will cause polarity within the molecule. This cyclopentanol is a polar molecule. What about ethene? That's actually a linear molecule - it might be diffucult to tell - but it's actually linear it's not bending at all it's a straight line going across. Alright now you've got a straight line going across are the sides the same? Do I have the same symmetry on both sides? And the answer is yes. This is a nonpolar molecule. So once again guys, the guide - the goal today - was to discuss molecular polarity; to introduce you to the idea that the molecules can either be polar or nonpolar but what I really want to key in on was that molecular polarity can primarily be determined by the symmetry of the shape. That is the number one thing. How polar it is though is going to be determined by the bond polarity. You have to be careful there the bond polarity is very important, although I don't cover it here. The symmetry will tell you whether it's polar or not. The bond polarity can really tell you how polar it is and how big of a dipole moment that has. Alright guys, well thanks a lot for tuning in I hope this was helpful. Be good.

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Duration: 14 minutes and 17 seconds
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Language: English
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Posted by: christineward on Aug 24, 2015

Polar and Non Polar Covalent Molecules, Polar vs. Nonpolar - CLEAR & SIMPLE

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