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Polarity - Concept
M.Ed., Columbia Teachers College
Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.
The electronegativity of the atoms involved in bonding determines the polarity of the bond. Electronegativity is a quantitative measure of how tightly an atom holds onto its electrons. Polarity has a great effect on solubility and other chemical interactions because polar and non-polar molecules do not like to interact.
Alright so let's talk about polarity in polar molecules and polar bonds and what that means, we know polarity means that unequal sharing of electrons so we talk about covalent bonds are actually sharing these electrons between them those valence electrons but sometimes they're not equal sharing, sometimes one element is actually going to share those electrons more than the other. They're actually going to be electron hogs versus electron givers. Okay so electro negativity is it's playing a major part in this, so it's a big thing so it's the ability to attract electrons and we know the trend of electro negativity is that fluorine is the most electronegative atom on the periodic table. Has electro negativity value of 3.98 now this is actually all in comparison these aren't actual values that's why there's no unit. These are actually in comparison with fluorine and the most electronegative atom. So francium has electro negativity value of 0.7 significantly less, this is the least electronegative atom in the periodic table this is the most electronegative atom. So how does that actually play a part in polarity?
Well need for this things bond one of these, the fluorine in this case will take all the electrons and hog them because they love electrons. These guys, high electro negativity means I'm an electron lover, this guy doesn't like electrons so wants to give them up. So how do they come together, so when you come, when you have a molecule that is, nitrogen for example that is, the 2 elements are the same they actually do have an equal sharing. This nitrogen is going to have electrons around it the same amount of time this nitrogen is. This is going to be a non-polar bond, however when you give it a carbon monoxide the value of the electro negativity value for carbon is 2.55 where the electro negativity value for oxygen is 3.44. This guy is much more electronegative, much more of an electron lover so all the electrons are going to crowd around the oxygen so we're actually going to denote that.
All the electrons are going to go this way, making this guy slightly negative and this guy fully positive and the way to denote that is we're going to have the symbol this has to be slightly negative, this guy can be slightly positive that's a lower case delta to represent this slightly negative or slightly positive charge, so because all the electrons are coming around oxygen. Okay great so that's a polar bond, but how do we know that, that makes it polar? Well we have to look at the electro negativity difference, the difference between the electro negativity values of those 2 elements in the bond. If the value is greater than 1.7, it's actually going to be ionic meaning there're actually not going to share these electrons anymore. One is actually going to take from the other one, so they're extremely electronegative, the difference between them is very, very great. So which is instead of sharing they're just going to transfer.
A polar covalent bond is between 0.4 and 1.7 with the difference between the 2 electro negativity values. If it's non-polar it's going to be pretty much the same, they're actually going to have either no difference or up to 0.4 difference if it attracts the negativity valued. And that would be a non-polar meaning they're pretty much equally shared between 2 atoms. But then how does this play a part when you're talking about molecules, polar molecules? Well let's take water for example, water we know it's a polar molecule. If you take the shape of water, we know it's a bent molecule we have these electrons they're unshared pushing these hydrogens down. This is a non-symmetrical; it might look symmetrical if you put a line this way.
However, we're going to say it's not symmetrical because overall the whole picture is not a symmetrical picture. If you put a line this way, it's not the same up and down, so we're going to say it's polar those actually an uneven pole. The electrons are going to go towards the oxygen making this end of the molecule slightly negative and this end of the molecule slightly positive okay. This is what we call a polar molecule, now if we go over to here to carbon tetrachloride this is as you can see very symmetrical. We're going to say overall the electron distribution between this molecule is even, there's no one side being more polar than the other. However if you look at just the carbon chloride bond, we can say the difference between this is pretty great, this is a polar bond so you can have polar bonds within a polar molecule. That does happen, it will exist, this is an example of that.
If you look at the shapes of your molecules you might see, there're ones that is always going to be polar no matter what because these electron are going to be bent for sure, bent will always be polar another thing that will always be polar is trigonal pyramidal you don't have to think about it. Those 2 always polar because those have electrons, otherwise you're going to have to take and analyze what exactly is, if they're different ones for example, if this was I don't know let's say bromine this would then be polar because then there's, this is different, this is not symmetrical these guys are different from these guys. So you actually have to look at those, to see if it's polar or not.
And polarity is a big deal when I know something is polar or not because they do it like, the reason oil and water don't mix for example is because of polarity. Water is polar oil is non-polar. So when you get to these things in more advanced topics, this polarity issue will definitely come up again.
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