Electronegativity and Polarity

Ajouté : 2026-05-09

[00:00:03]all right guys um so we're going to kind of switch gears um and we're going to start looking at periodic trends um these are going to be different kind of properties the elements and compounds will have based off of their location on the periodic table and with that it's going to lead us into looking at polarity which we mentioned earlier this week um but we'll be able to determine if a compound is polar or nonpolar based off of some of these trends our first Trend we're going to look at is electronegativity electronegativity is the ability of an atom of an element to attract electrons when the atoms are in a compound so if something has a high electronegativity it has a very strong pull on those electrons if it's a weak electronegativity or a lower electronegativity it has a weak pull on the electrons the overall trend for electronegativity is that it decreases from top to bottom in a group so going down a row on the periodic table it gets smaller or excuse me a column and it increases from left to right across the period so these wedges will kind of make sense a little bit more later but overall that means that our high selection negativity is going to be towards that top right corner is going to be the highest now with electronegativity it said that it has to attract electrons when atoms are in a compound so when bonded our noble gases remember are stable and they tend to not Bond so this does not include noble gases so on the periodic table the element that's in that top right corner that is not a noble gas is fluorine so fluorine is the element that has the highest electronegativity this can kind of come up we've seen it in the past on where we had state test um this would be a trick question where they would ask which element is the highest selection activity and they would have both fluorine and helium but you had to remember that helium was a noble gas so did not have electronegativity value yeah so how this relates to polarity um since we said that some atoms can attract electrons more if they're more electronegative we end up where electrons are not always shared equally in that covalent bond so if two substances have about the same electronegativity they can end up making a nonpolar covalent bond where the electrons in the covalent bond are shared equally between the atoms all of our diatomic molecules that we talked about those are nonpolar um because like oxygen O2 one oxygen is not going to be more electronegative than the other oxygen so they are going to share equally um the nonpolar covalent bonds and these also tend to be very symmetrical since the electrons are shared equally between the two atoms what we also end up with are polar covalent bonds or polar bonds these are covalent bonds between atoms that do not share electrons equally so atoms that have different electronegativity values are not going to share electrons equally one of them will be more electronegative or stronger than the other and kind of hog all of the electrons these are going to end up being non-symmetrical with one side being larger or having more electrons than the other the more electronegative atom is going to attract electrons more strongly and gain a slightly negative charge and we denote a slight negative with that Delta negative sign um and the less electronegative atom has a slightly positive charge with that Delta positive sign um so you can kind of see this if you look at the image in the bottom left um it does show you that our diatomic ones where we have like our two hydrogens and two chlorines they are truly balanced one of them is not more electronegative than the other it is going to end up being symmetrical the blue circle is kind of showing the electron cloud where all of the electrons are in that place however if you combine a hydrogen and chlorine chlorine is towards that top right corner whereas hydrogen is towards the top left corner chlorine is more electronegative than hydrogen so chlorine is going to kind of hog all of the electrons ending up with the chlorine side being larger than the hydrogen side so it's kind of hogging all of those electrons the table on the right you'll see one similar to this in your homework um this is just showing you each element and its electronegativity value um so where we would normally have the mass on a periodic table here it has the electronegativity value um so you can see that like fluorine has the highest selection negativity whereas francium in that bottom left corner is the lowest electron activity value so you can kind of see that gradual increase going from the bottom left corner to the top right corner and what we do with these numbers those electronegativity values we can subtract them to find the electronegativity difference and that's going to tell us what type of bond we're dealing with um and this is something that you will do on your homework so when we subtract that electronegativity if the difference is anywhere from 0 to 0.4 that's a very small electronegativity difference they're pretty much going to be the same electronegativity um and that will end up resulting in a nonpolar covalent bond um and make sure that for it to be a non-polar covalent bond it has to be made of two non-metals you'll see that there are some exceptions where it might be a metal in a non-metal that are very close together but a metal and a non-metal cannot make a nonpolar covalent bond um so nonpolar can only be covalent bonds polar can also only be covalent bonds so if when we subtract the electronegativities it's greater than 0.4 but less than or equal to that 1.7 there's a good different selection activity there um that is going to end up in a polar covalent bond and yet again it has to be between two non-metals now if it is greater than 1.7 it tends to be ionic and remember ionic is going to be between a metal and a nonmetal like I said we can do the electronegativity difference but these ranges are not always going to work um simply because you might have two non-metals that have very different electronegativity values but they're still considered polar because they're two nonmetals or we might have a metal and non-metal where the difference is less than 1.7 but a metal and a non-metal can only be ionic um so let's look at one of these I'm going to go back a slide that way we can look at those electronegativity values um so if we were dealing with um let's say like CF would be a bond that we're dealing with we would subtract the two so fluorine those numbers are kind of tiny but fluorine is 4.0 carbon is 2.5 so if we subtract those that is 1.5 is our electronegativity difference so this would be a polar bond because it is between two non-metals um and it's that kind of between 0.4 and 1.7 um whereas if we had sorry I'm trying to find a good example if we had like chlorine to bromine I'm going to do this one at the bottom chlorine is 3.0 bromine is 2.8 so that's a 0.2 difference so that would be a nonpolar covalent bonds so you're simply doing a subtraction of those electronegativity values and seeing which range it falls in but like I said make sure to double check if it is two non-metals it's either polar nonpolar if it is a metal and a non-metal it is ionic even if it does not match up with the range thank you so with the presence of those polar bonds this can make our entire molecule polar so if you remember where we talked about properties of molecular compounds or covalent bonds we said they can be polar in nature so polar molecule we're going to end up with one end of the molecule being slightly negative and one end of the molecule being slightly positive so because the electrons are not shared equally we end up with a negative end and a positive end similar to like a magnet would be um these these two ends can act as poles so like a magnet and we say that they will have what we refer to as a dipole so dye means two so this is going to be a molecule that has two poles so a molecule that has a positive end and a negative end and these tend to react with other polar molecules or even magnetic fields they connect with as well so when polar molecules are placed between oppositely charged plates or magnets and they tend to become oriented or aligned towards the pause of a negative plate so all of the positive ends of the molecules will kind of be facing the negative magnet whereas the negative ends of molecules will all face the positive magnet since those opposite charges attract one another and how we tell if the molecule as a whole is going to end up being polar there are a couple things that we look at first we look and see does this molecule have polar bonds so does it have um atoms that have that different selection activity if it does have polar bonds we then have to look at its molecular geometry or shape and see if the shape prevents polar bonds from canceling um so polar bonds would need a pull against other polar bonds kind of like a tug of war to cancel out and there are certain shapes that tend to allow for the molecule to be polar versus nonpolar so we're going to relate this back to your Vesper shapes that way you can see if it's polar nonpolar so our geometries that allow four polar bonds to cancel these are going to be the shapes that are also molecular geometry so the ones without dots on the center tend to be nonpolar now I say tend to be because there are always exceptions to the rule and it's like something that's linear so one thing that we have drawn that's linear is carbon dioxide and I'm just going to sketch it real quick so here we can see if we're looking at Carbon and oxygen oxygen is further to that top right corner so oxygen is more electronegative so all of the electrons are going to be pulled towards the oxygen from the carbon that's what you see these arrows representing well the other oxygen is also kind of pulling electrons from the carbon so these two are able to pull against one another allowing those polar bonds to cancel out so this ends up being nonpolar for trigonal planar same kind of thing applies um if there are not dots on the center atom it allows all of those to pull against one another and cancel out same thing with tetrahedral where I said that there are exceptions to everything I'm gonna put on here four nonpolar outer atoms must match so it is possible that you have something linear that is polar if those outer atoms do not match um so I'll draw like hydrocyanic acid or hcn looks like this whenever we're looking at electronegativities carbon it's a slightly higher electron activity than hydrogen but nitrogen is more electronegative than carbon so all of the electrons are kind of being pulled towards the nitrogen so since those forces are not pulling against one another this would end up being polar even though it's linear so these are kind of General shapes that tend to be nonpolar but like I said if those outer atoms do not match it tends to end up being polar instead and then we get to the shapes that do not allow polar bonds to cancel these are going to be the ones that have dots on Center atoms or the ones where the outer atoms don't match so like if we have a bent structure so water where we drew it looks like this well if we are looking at those electronegativity values oxygen is more electronegative than hydrogen so the electrons are being pulled towards the oxygens but notice that since they're bent they are not straight across from one another so they are not pulling against one another and there's nothing pushing in the opposite direction so it ends up being polar same thing with trigonal pyramid uh one of the things we do that was trigonal pyramid was ammonia so looking here nitrogen is more electronegative than hydrogen so all of the electrons are being pulled towards the nitrogen but there is nothing going in the opposite direction from that electron pair so that ends up being polar as well with those not being able to cancel so once you draw the Dot Structure you can kind of guess if it is polar or nonpolar so I would first check and see are the outer atoms all the same and if it is that linear trigonal planar tetrahedral with all atoms the same they're not dots on the center atom it tends to be nonpolar whereas if there are dots on the center atom or those outer atoms are different it is going to be a polar structure all right and with our molecules that are polar nonpolar a lot of times this goes into solubility and looking at if substances will dissolve in one another polar molecules are only soluble or can only be dissolved by other polar molecules so polar molecules can interact with other polar molecules nonpolar molecules will interact with non-polar molecules so you may have heard the term like dissolves like um how this works um the picture in the Middle with the blue background if you've ever washed your own dishes or done your own laundry you'll kind of know that if you do not use soap it doesn't get clean same thing wherever you are trying to wash your hands if you don't use soap you're not actually cleaning your hands and that's because soap are the little black structures they have a polar head and a non-polar tail similar to your lipids in your phospholipid bilayer that you talked about in biology that had that polar head and the nonpolar fatty acid tail your polar head of the soap will attach to water but the nonpolar tail will attach to the dirt and oil so whenever it rinses and the water drains it actually pulls the oil from the dishes or the clothing and we're going to stop here and continue with the rest of the trends tomorrow or excuse me on Monday as well as looking at the intermolecular forces that occur between molecules