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Polar Covalent Bonds
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0:01
>> host: Welcome back to Chemistry Matters, and the third video on our Unit 4 playlist: Chemical Bonding. We've investigated some of the characteristics of intramolecular bonds, specifically ionic and covalent bonds, which sit at either end of the bond spectrum we discussed in the second video for this unit. Now let's go back out to our classroom where our students are learning about types of bonds that fall more in the middle of the bonding spectrum.
0:30
>> professor: Remember earlier how we said it's more accurate to think of different types of bonds as falling along a spectrum, as opposed to being one specific type? Well, let's explore a type of bond that falls right in the middle of that bond spectrum-- a polar covalent bond.
0:51
The reason polar covalent bonds fall into the center of the bond spectrum in between ionic and covalent is because the atoms that participate in polar covalent bonding have electronegativities that are different from each other. But there just isn't a large enough difference for an electron to be completely removed from the atom with the lower electronegativity. The compound you're most familiar with that has polar covalent bonds is water. In a water molecule, we say the electrons that are
1:19
being shared between the hydrogen atoms and the oxygen atoms are being shared unequally. What we mean is that the oxygen has the greater electronegativity, so it tends to keep the electrons near itself. Since the electrons tend to stay near the oxygen more than they do the hydrogen, we say the oxygen end of the molecule develops a partially negative charge. Meanwhile, think about the hydrogen atom. What did it start off with in terms of electrons and protons? >> student: one proton
1:48
and one electron, right? >> professor: Exactly, but if its one electron is spending most of its time around the oxygen atom, then what do you have left?
1:58
>> student: Just a proton? So does that mean the hydrogen ends of the water molecules are partially positive? >> student: Yeah, it's basically just an exposed proton? >> professor: That's exactly right! And that's what makes the water molecule polar. It's similar to the way a bar magnet has a north and south pole. Polar molecules have a partially positive and a partially negative end. And these types of molecules are characterized by chemists as containing dipoles, which simply means
2:26
that the molecule has two poles, similar to a bar magnet. And in the same way that two bar magnets stick together when brought close together polar molecules will also stick together, with the partially negative end of one molecule attracting to the partially positive end of the other molecule. Now, let's compare the polar covalent bond to a nonpolar covalent bond, like we explored earlier with the hydrogen molecule.
3:01
Take a look at this image of a chlorine molecule, which is two chlorine atoms bonded together. Their electronegativities are identical, so there's no way there can be a buildup of electrons on either side of the structure like we saw with the polar water molecule. It's kind of like a tug of war battle in which the opponents are evenly matched. The rope stays taut in the middle-- keeping both sides together. That's how a nonpolar covalent bond works. The electrons spend equal time
3:29
with both atoms, so equal attraction to each atom.
3:33
>> host: By now, you have a solid, general understanding of how intramolecular bonding works. You should also have a better sense of the various types, or ranges of bonds that hold matter together to make up our physical world. So now we're going to learn how to predict what kind of intramolecular bond a substance has, and where that bond would lie along the bond spectrum. This activity will help you visualize the types of bonds that form as atoms come together to make molecules and compounds.
4:06
For this lab, our students will use a simulation of bonding that you can find in our Chemistry Matters Toolkit. There you'll find instructions on how to form bonds between two or three atoms and how to use atoms with a wide range of electronegativities to form different kinds of bonds. The simulation will allow you to conduct your own experiments and collect data about the types of bonds you're forming.
4:31
So let's return to our classroom where our teacher and students are getting some practice with this bonding simulation activity.
4:40
>> professor: Look at the atoms, A and B. Above them you can see slider boxes that show whether the atoms have high or low electronegativity. Notice that I have set the slider bars so that atom A and Atom B both have the same high electronegativity value. Below the atoms you can see a color-coded bar, which is kind of like the legend you find on map. This color bar is a key for understanding the meaning of colors in the electron cloud drawing above. Blue indicates an area
5:08
with strong positive charge. Red indicates an area with strong negative charge. And the white bar color indicates no strong charge. Which is why the electron cloud in this image is white. Your job right now is to look at some images I've created by adjusting the slider bars, like this one, and use the data you see to figure out what slider bar combinations produce ionic bonds, which ones produce polar covalent bonds, which ones produce mostly covalent bonds, and which ones produce
5:38
non-polar covalent bonds. So let's start with this bond image. I want to see if you can tell what kind of bond it is. Look at the two atoms, A and B. Does this image show a partial positive or negative charge in this molecule?
5:53
>> student: It looks like it doesn't have a charge. The circles around the atoms are white. >> student: So what does that mean? >> professor: Remember, the white electron cloud around the molecule indicates there's no strong charge on the molecule. >> student: Yeah, under the slider bars, the molecule is identified as being "more covalent." That makes sense because both atoms are pulling hard on the shared electrons. >> professor: You're correct. The bar labeled bond character
6:19
tells us that it's a more covalent bond. A molecule like this, remember, with no strong partial positive or negative charge, is a nonpolar molecule. So this is an example of a nonpolar covalent bond.
6:34
>> host: Our students have gotten a nice start identifying and predicting types of bonds. Now they're going to continue this activity by moving the electronegativity sliders themselves to create different types of bonds. This will help them gather data and identify patterns in the formation of intramolecular bonds. A lot of students have found this to be a very helpful exercise for solidifying their understanding of bond types. So before continuing with the rest of this unit,
7:05
now is an excellent time for you to get some practice too. In our Chemistry Matters Toolkit, you'll find the information needed to perform this exercise. You can use simulations just like our students used, and even manipulate the properties of the bond types by adjusting the electronegativity sliders. When you start the simulation, make sure you click on the Bond Character button in the View box and the Electrostatic Potential in the Surface box. This activity lets you explore how the nature
7:39
of bonds that form between atoms is directly impacted by the electronegativity of the atoms. You'll get to choose how many new molecules you form. No matter how many new molecules you choose to form, make sure you write down the electronegativity and the bond character for each bond you create. Then you'll be able to look for patterns in your data.
8:02
Once you've practiced identifying and predicting types of bonds, rejoin me for the next video on the Unit 4 playlist where we'll take a look at the results our students found with this activity. I'll see you there!
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