Polar Molecules Tutorial: How to determine polarity in a molecule

Polar Molecules Tutorial: How to determine polarity in a molecule


in which we will explore how bond
polarity affects the polarity of a molecule so what is polarity?
First molecules are neutral; the entire molecule has an equal amount of protons
and electrons; with that in mind if the negative charge, the electrons in a
molecule, are evenly distributed around the molecule, then it is not polar.
however if the negative charge is not evenly distributed, then the molecule is
polar. Water is a well-known polar molecule. electrons are moving very fast
but the more electronegative oxygen pulls electrons away from the less
electronegative hydrogen. In this color-coded illustration the more red area has a greater concentration of electrons so the oxygen side of water is more negative while the blue area has a lower concentration of electrons so it is more positive. water has an uneven distribution of
electrons, it is a polar molecule. but why does this occur? we will take a closer look at water and a variety of other molecules to see what determines whether
a molecule is polar, including its and we will go through many examples of this we will go through several examples demonstrating these concepts and then put them in a flowchart. Let’s take a close look at water and see why it is a polar molecule. first of all it is bent, meaning it does not have a symmetrical shape this is important, and here we will define symmetrical as having the central atom here is oxygen and both hydrogen’s are on one side of the oxygen. which makes it non-symmetrical. the hydrogens are not evenly distributed around the oxygen. next we look at bond polarity by the difference in bonded atoms’ electronegativities. in general electronegativity values increase across the periodic table from left to right
and from bottom to top. we see here that hydrogen has an electronegativity of 2.1 and oxygen has an electronegativity of 3.5 the difference between 3.5 and 2.1 is fairly large, meaning the hydrogen-oxygen bond has a large polarity which is shown with an arrow pointing towardthe more electronegative atom in this case oxygen if we view the bond polarities as vectors and add them together the resulting vector, or arrow,
indicates the polarity of the molecule. the blue arrow is called a dipole, note these are partial charges, not the full charge that an ion would have let’s look at a less familiar molecule
sulphur difluoride with sulfur in the center. the Lewis structure shows two lone pairs on the sulfur and so it is a bent, and therefore non-symetrical molecule. the electronegativity differences in the fluorine-sulfur bonds show a large bond polarity, but this time pointing toward the outside atoms, the
more electronegative fluorines. adding the bond polarity vectors again gives a large dipole, with sulfur partially positive and the fluorines partially negative. next let’s look at carbon dioxide with carbon in the center. the Lewis structure shows each carbon-oxygen bond being a double bond. the shape of CO2 is linear triatomic which is a symmetrical shape. the oxygens are evenly
distributed around the carbon. there is a fairly large electronegativity
difference in the carbon-oxygen bond. So will there be a dipole? what will happen when we add the bond polarity vectors? that’s right! they add up to zero, they
cancel each other out so there is no dipole. I hope you can see the importance of symmetry here. the symmetry is why the bond polarities cancel. now let’s make the outside atoms different. How will that affect polarity? here we see the electronegativity differences in the bonds, and thus the bond polarities are different. what will be the result of adding them together? that’s right they add up to a nonzero vector. SCO is a polar molecule with the oxygen side being more negative the sulfur side being more positive. Let’s do a couple more examples, and then put all this information together in a flow chart. boron trifluoride with boron in the center the fluorines are equidistant from each other at 120. the electronegativities show a large difference in the B-F bond resulting in a large bond polarity in each bond. however if we add up the bond polarity vectors they add up to zero. the bond polarities cancel due to symmetry and so there is no dipole. let’s do something interesting. let’s replace one of the fluorines in BF3 with a chlorine, BF2Cl. but the electronegativity differences for each
bond are no longer the same. The B-Cl electronegativity difference is
only half that of the B-F bond. adding the bond polarity vectors
results in a non zero vector. which is the dipole. everything we have been looking at so far has been the result of combining
bond polarities with the molecule shape and identity of the outside atoms to
determine molecule polarity. Let’s put that in a flow chart. you begin by looking to see if any bonds are polar and if not then you cannot have a polar
molecule. for the purposes of determining molecular polarity we consider an electronegativity difference greater than zero to be polar. now if there are polar bonds then you look at the symmetry of the outside atoms which is
determined by the shape of the molecule. symmetrical shapes are outside atoms are evenly distributed
around the central atom. non-symmetrical shapes are and we include diatomic as a special case which we will get back to momentarily. let’s keep that list in the corner for reference. the rest of the flow chart summarizes the examples we have just looked at. if polar bonds are not symmetrically distributed, then the molecule is polar. if they are symmetrically distributed then you have to look at the outside atoms if they are the same that means the bond polarities cancel but if the outside atoms are different that means the electronegativity differences are giving different magnitudes of bond polarities and so they do not cancel. let’s take a quick look at diatomic molecules, our examples being HCl and Cl2 For HCl is polar and so the molecule is polar. for Cl2 there is no bond polarity. lastly we will take a brief look at how changes in a tetrahedral molecule affects polarity. In methane everything is symmetrical bond polarities cancel and it is nonpolar. But if we substitute a fluorine for one hydrogen then both the magnitude and direction of that bond’s polarity changes the bond polarity vectors add up to a large dipole so CH3F is polar. changing another hydrogen to fluorine
has the same effect on bond polarity. again creating a dipole but with a slightly different direction changing a third hydrogen to fluorine we get the same effect, the polarity changes direction but it is still polar. What will happen to the polarity if we change the fourth hydrogen to fluorine? yes you’re right since all the outside atoms are the same the symmetry results in bond polarity vectors adding to zero they cancel each other and carbon tetrafluoride is a non-polar molecule.

100 Replies to “Polar Molecules Tutorial: How to determine polarity in a molecule”

  1. This video really helped me alot to understand this. At a first look,this looks very complicated and complexed but it is actually pretty logical and easy if you understand the basics.

  2. Now I get it! This is probably about the 5th video i've seen on polarity and this one is the only one to has it make sense for me. Thank you Crash Chemistry Academy!

  3. This is so awesome. I understood from beginning to end, something sorely missing than the other vids I looked at. Very clear, and your illustrations really nailed it. Thank you!!!!!!!!!!!

  4. How to make the electronic circuit to measure the dipole moment, with reference to an article from 40 years ago in the magazine "Journal of Chemistry Education" (USA). Link is https://youtu.be/Xz3WIcEvxFM (15 feb 2018).

  5. This is the definition of gold. I have watched so many videos to find out how to determine if a molecule is polar or not polar, and this is soooooo clear. A massive thankyou! I sent this to my teacher so she could send it to all my classmates so they too can understand how it works. 🙂

  6. At 7:19 the blue arrow points toward one fluorine atom instead of between them as 7:23 shows from off the Boron atom. Plus the red triangle arrows appear incorrect pointing counter clockwise. Anyone elaborate?

  7. You know your real life chemistry teacher sucks at teaching when you have to watch YouTube to understand what she said. Great video by the way

  8. Your animations and explanations are excellent specially hybridization video. Thank you. The only point I would make in the polarity lesson is regarding ozone, which is polar. The definition of polarity is based on what you stated at the very beginning, which is asymmetrical distribution of electron density around the molecule. Because ozone has a lone pair and is bent, the electron density is not evenly distributed. It does not matter if O-O bond itself is non-polar. Dipoles will not cancel out.

  9. AHHH I was freaking out last minute cause I didn't understand this in my Chem 20 class. Thanks, this cleared up the confusion!!!

  10. Wow, I reviewed this several times and in just a short while and understand so much. Our whole class is a bit confused because of our chemistry teacher, is doing a terrible job. He wings it in class, is disorganized and unprepared and explains thing in pieces and jumps around. Maybe Ph.D.'s should continue to refine their methods to be able to explain it to us, laypersons. It's so sad that a professor can make so much money and we pay so much tuition and get a better education from youtube for free. FANTASTIC JOB TO THE AUTHOR HE UNDERSTANDS HOW TO TEACH.

  11. Teacher, 10:16 You could to pair both trigonal planar and pyramidal, in the illustration next time.

    trigonal planar has no eletric negative charge. Bu pyramidal has uneven distribuion of electric negative charge. Right?

  12. sir, your explanation is super clear to me. As u mentioned, CO2 is a nonpolar molecule since its polarity cancels out, but why does it absorb infrared and contribute to Green house effect , because as we kNow that only molecules that show polarity absorb infrared

  13. Thanks for your useful video. you said that ozone is not polar because ozone has not polar band but ozone is non symmetrical because of electrons around central atom can you explain more ozone is polar or not polar?

  14. Sometimes I doubt if I really have to go to school. YouTube makes lessons easier, more than a two-hour classroom discussion. This one saved me for my Chem exams

  15. I am a math teacher and assign to teach physical science where first part is chem. Thank you for this video. I can now teach it to my learners

  16. You have no idea how hard it was to find a video that actually explained this well. Yesterday I found a website that has very simple breakdowns of chemistry terms and now this! I'm on a roll! Thank you thank you thank you 😀

  17. Brilliant explanation. What I didnt understand in 10 chemistry classes was explained to me easily in 10 minutes

  18. What would you say if I told you my professor told our class that a bond is non-polar if it is less that 0.5 and not greater than 0?
    Is this technically okay? Should I just play by the professor's rules?

  19. what about the compound CH3I? is it polar? I'm asking because it has the iodine atom that has a similar electron-negativity of carbon, but it has a higher electron-negativity of hydrogen.

  20. If their is a option of liking 10000 times i will click that . thank you sir for giving such a good explanation.
    ??

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