9.2 Polarity

Question 1

Electronegativity
* F is the most electronegative, the closer you get to it (going from down to up or from left to right) the more electronegative the atom becomes (w/ left to right carrying slightly more weight)
* F is # 1
* O is # 2
* Theres some debate between N and Cl

Polar bonds
* two identicaly atoms bonded = non polar because they have the same electronegativity (so one won’t hog the electrons more than the other = non polar)
* carbon and hydrogen bonded together are non polar as well [both have very low electronegativity so they’re non polar]
* if the electronegativity delta between the two atoms is less than 0.5 (dont memeorize) its considered non polar (one doesnt really pull harder than the other = nonpolar)

Answer 1

Question 2

Which of the below has the biggest poarity?

C-F
C-Cl
C-Br
C-I

Answer 2

Well all were looking for is the biggest electronegativity difference between atoms
* since C is all the same (low electronegativity) all we have to do is pick the most electronegative atom (which will hog the electrons) to make it the most polar (meaning the carbon will have a partial positive, while the F will have a partial negative)

C-F is the answer

Question 3

If a molecule has no polar bonds (less than that 0.5 delta) its considered a non polar molecule

if a molecule has polar bonds, it may be polar, it may end up being non polar. Its going to depend on what the polar bonds, and how they’re oriented around the central atom (they might cancel out –> the net dipole adds up to 0 [would be called a 0 dipole moment])

Below:
* the carbon oxygen bonds are defiently polar (the electronegativity delta is >0.5)
* The arrow sign drawn at the top is pointed toward the more electronnegative atom
* can also write partial charges at the bottom
* Keep in mind electron geometry, in this case lewis was right, this is linear
* If we’ve got = bond dipoles (which we do because they’re both carbon oxygen double bonds), and they point 180 degrees apart. Well if you added these as vectors, they would add up to 0. So we would say this molecule is nonpolar over all, even though it has polar bonds, but overall its non polar. Has no overall dipole
* So we would say this is nonpolar
* We could talk about pulling a rope on that carbon. If we pull from 2 opposite and equal directions. The question is would the carbon move anywhere. In this case it wouldnt, so the carbon isnt going to be pulled, net, in either direction. And thats one way of saying the dipole adds up to 0 (the carbon isnt pulled anywhere)
* If you get a 0 dipol moment its polar
* The greater the dipole moment, the more polarity

Answer 3

Question 4

Looking below at the polar SCO molecule

So in the molecule below we still have 2 polar bonds (the have greater than a 0.5 difference in electronegtaivity, carbon is less electornegative while sulfur and oxygen have increased electornegativity)
* however, the carbon sulfur bond is much less polar than the carbon oxygen bond, so we draw a smaller arrow
* so we have 2 bond dipoles, that are 180 degrees apart, however, they arent equal in magnitude, so they dont add to 0. So we have an overall dipole moment pointing towards oxygen.
* So the overall molecule dipole would point to the right (drawn in blue at the bottom) because when you add the two red arrows, it points towards the right

being polar really impacts the properties of these molescules, in terms of things like boiling points, melting points, viscosity, etc…

so the first thing to do is look to see if you have polar bonds. If you do you need to see how they’re oriented and their oritened and their magnitude. If they cancel (net dipole of 0) its considered a non polar molecule.
* However, we have to be really careful of lewis structures because thats not the actaul molecular geometry. Need to follow those VSPR rules.
* However, we the linear ones we looked at it worked.

Question 5

Comparing Lewis structures vs actual geometry

Looking at the lewis structure below lots of students say its polar even though it isnt.
* were told hydrogens are non-polar, so we dont even have to worry baout those
* Cl is polar, but its polar in equal and opposite directions w/ the other Cl atom, so it looks like its non-polar
* However, they arent equal and opposite directinos, this is a tetrahedral orientation, meaning they’re pulling at 109.5 degrees from each other, not 180 degrees
* think 2 ropes pulling on carbon at 109.5 degrees from eachother, that would make it non-polar

When you add those 2 vectors 109.5 degrees apart, you get the average of them (blue line) which is right in the middle of the 2 vectors.
* so the overall dipole moment is shown in blue

NOTE: when you have different atoms around the central atom, it usually results in it being polar, because the dipoles won’t typically cancel out completely

Question 6

If you’ve got all identical atoms around the central atom, and that central atom has no lone pairs, then your bond dipoles will cancel, and it will be non polar

BCl: Nonpolar trigonal planar
* All the same atoms w/ no lone pairs around the central atom = non polar
* the average of all the polarites = 0 = non polar
* if you pulled on that boron in 3 different directions, all 120 degrees apart, it wouldnt go anywhere = non polar
* vector sum = 0
* remember, lewis structures draw everything at 90 or 180 degrees apart, so if you drew the lewis structure for this guy it would look like it had polarity, but in reality it doesnt

CF4: 4 atoms w/ no lone pairs pulling on the central carbon = non polar
* since its all the same atom they cancel eachother out
* Think about a carbon being pulled on at the same magnitude in 4 places 109.5 degrees apart, it would go no where = non polar
* vector sum of 0 = non polar
* identical atoms around a central atom w/ no lone pairs = non polar always

SO2: So the central atom has a lone pair and you have 2 oxgen atoms
* this one is bent because 1/3 of the electron domains is a lone pair while the other 2 are bonding.
* They’re a little less than 120 apart, equal in magnitude but not opposite in direction, gives us a dipole on the molecule (the lone pairs don’t pull at all)

NF3 molecule: So this has 1 lone pair and 3 F atoms on it. meaning it will be polar becuase that lone pair doesnt pull at all while the F’s do
* So the F’s are pulling in 3 different directions at 109.5 degrees apart on this Trigonal pyramidal atom. They won’t all cancel out because they dont have a 4th 1 pulling at 109.5 degrees apart, leading to a net sum = some dipole

H2O molecule. Look at the lewis structure above. That makes this really tricky because it looks like it should be non-polar, however, when looking at the real molecular geometry it is polar.
* The hydrogens are no electronegative but the oxygen is so the electrons are pulled back to the oxygen. The oxygen is pulling in 2 places at 109.5 degrees, making it polar (don’t cancel out)

Question 7

So its not a hard and fast rule that when there are lone pairs on the central atom its polar. I’m going to show some examples of when it remains non-polar but there are lone pairs on the central atom

XeF4
* lone pairs always on the top and bottom of square planar so they’re 180 degrees apart instead of 90
* Now all the F atoms are pulling at 90 degrees apart = non polar

XeF2
* The atoms are pulling at 180 degrees apart, and they have the same magnitude of electornegativity = non polar (pulling in opposite directions at the same magnitude cancel out)
* The lone pairs must be on the equator spots because they only have 2 within 90 degrees of them instead of 3

Rule if you don’t have lone pairs = non polar
* Rule is not that if you have lone pairs you’re automatically polar (this is untrue in these 2 examples, but is often true)

So the linear w/ 5 domains, and the squayre planar w/ 6 domains are the only exceptions to the rule. If its not one of these 2 and has lone pairs it will be polar (shown below)