Polarity of Molecular Structures

Dipole moments or polarity happen in molecules when there is an unequal sharing of electrons. 

Molecular Polarity

Concept: Understanding Molecular Polarity 

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Video Transcript

We're going to say molecules that have unequal sharing of electrons contain a molecular polarity. Why do they have unequal sharing? They have unequal sharing because one element will be more electronegative than the other elements. That element that is more electronegative will hoard or take in all the electron in for itself making it more partially negative. The other elements as a result will become more partially positive. What we're going to say here is to do this, we're going to have basic rules we have to follow. We’re going to say in these molecules, the reason that they're polar is because of their shape and also because of bond polarity.
Recall, just the shape and the bond polarity will affect the whole polarity of the molecule. This has to do with some of the concepts we learned about electronegativity and also some of the concepts we learned about dipole arrows. These are going to come into play when we're trying to figure out the polarity a molecule. 

Both a molecule's shape and bond polarity can affect its overall polarity. 

Concept: The First Rule for Molecular Polarity

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Video Transcript

We break it down into two simple rules. These are the rules to determine if a compound will be nonpolar. If it follows all the rules, it's going to be nonpolar. If it breaks any of them, it’s going to automatically be polar.
We're going to say for molecular compounds where the central element has no lone pairs, no lone pairs at all, we're going to follow two rules. We’re going to say first the central element must be connected to the same elements. That's rule number one. If it breaks that, it’s automatically polar. Then second rules, the central element must be less electronegative than the surrounding elements. If it breaks this rule, it's automatically polar. You have to follow both rules in order to be nonpolar.
Let me give you a quick example before I let you guys do this one as practice. We could have CCl4. We’d say the central element has the same elements all around so it follows rule 1A. Then we’re going to say the central element carbon is less electronegative than the chlorines around it. Therefore, this follows both rules, so therefore it’s nonpolar.
But let’s say I did carbon in the center and I do this instead. Now we're going to say that the central element doesn't have all the same groups around it so it breaks rule 1A and automatically, since it breaks any of the rules, it's going to be polar. This is the approach you have to take.

When the central element has no lone pairs and has the same surrounding elements then usually the molecule is nonpolar. 

Problem: Determine if carbon dioxide is polar or nonpolar.

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Example: Determine if silicon tetrachloride , SiCl4, is polar or nonpolar. 

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Concept: The Second Rule for Molecular Polarity

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Video Transcript

Welcome back, guys! We're now going to continue our discussion on polarity when our central element has lone pairs. It sucks when our central element has lone pairs because we have more rules we have to follow. Let's take a look. We're going to say when our central element does have lone pairs, the central element must be connected to the same elements. We're also going to say that the central element must be less electronegative than the surrounding elements. Those two rules stay the same whether we have lone pairs or we don't.
Here's the new part. We're going to say use dipole arrows to point to the more electronegative element. These dipole arrows must cancel out. Remember, we talked about dipole arrows a few videos back. For them to cancel out, they have to go in opposite directions. If they go inward, they don't cancel out. They actually make the molecule more polar, make the bonds more polar.
We're also going to say that we also have dipole arrows for lone pairs. We're going to say dipole arrows extend away from lone pairs. These long pier dipole arrows must also cancel out. We need to make a distinction here. We have two types of dipole arrows. We have dipole arrows between elements and we have dipole arrows between lone pairs. They cannot cancel each other out. Lone pair dipole arrows must cancel out lone pair dipole arrows. Element dipole arrows must cancel out element dipole arrows.
Before I let you do this one, let's look at an example together. Let's say we have to draw NH3. NH3 is nitrogen in the center and we have the hydrogens connected to it. See our central element has lone pairs, so we use the rules. The central element must be connected to the same elements, which it is. But it breaks rule 2B. The central element is more electronegative. Since it breaks rule 2B, this is automatically polar. You would say that this compound here is automatically polar.
Let's take a look at another example. Here we're going to say, let’s see. We have Sn, or actually we’ll have OF2. OF2 would look like this. First rule, we’re going to say central element must be connected to the same elements all the way around, which it is. Center element must be less electronegative than the surrounding elements, which it is. We have to use dipole arrows for pat C and D. We're going to say use dipole arrows to point to the more electronegative element. The way we do this is we look at the oxygen. Here's our oxygen. We’re looking between this oxygen here and this fluorine here. We're going to draw a dipole arrow between the two. Whoever is more electronegative between the two, we point the dipole arrow towards them. Fluorine is more electronegative so we point a dipole arrow towards it.
Now we’re going to look again at the oxygen and the other fluorine. Again, the dipole arrow will point towards the more electronegative element, that fluorine. We’re going to say that these two dipole arrows, element dipole arrows, are pointing in opposite directions. They're going to cancel each other out. Part C is followed as well.
Now we have to go to Part D. For Part D, lone pairs also have their own dipoles. We're going to have a dipole arrow extending away from this lone pair and away from this lone pair. We're going to say that both those dipole arrows are pointing in opposite directions away from each other. Because they're pointing away from each other, they're going to cancel each other out. Part D is also followed. Using Part A, B, C and D, you see that we follow every single rule. Therefore, we’ll say that this is nonpolar. That’s the steps that you have to take when answering these types of questions.
I want you guys to take the same approach when you draw xenon tetrafluoride. Draw it out first. See if it has lone pairs. Use the rules from A to D to see what the correct answer would be. Once you do that, click on the explanation button and see what we do to figure out if it’s polar or nonpolar.

When the central element has lone pairs then we must use element dipole arrows and lone pair dipole arrows to determine polarity. 

Problem: Determine if xenon tetrafluoride is polar or nonpolar.

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Example: Determine if phosphorus trihydride, PH3, is polar or nonpolar. 

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Problem: Determine if the following compound is polar or nonpolar.

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Problem: Determine if the following compound is polar or nonpolar.

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Problem: Determine if the following compound is polar or nonpolar.

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Problem: Determine if the following compound is polar or nonpolar.

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Polarity of Molecular Structures Additional Practice Problems

Which of these molecules are polar? Check all that apply

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An unknown substance dissolves readily in water but not in benzene (a nonpolar solvent). Molecules of what type are present in the substance?

  1. neither polar nor nonpolar
  2. nonpolar
  3. either polar or nonpolar
  4. polar
  5. not enough information given
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Which of the following will be dissolved in water?

NH3, CF4, CO2

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Select the molecule that is the most polar
a. PCI5
b. BrCl5
c. XeF4
d. CCl4
e. XeF2

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Explain why CO2 is nonpolar, but OCS is polar?

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In which of the following liquids is iodine (I 2) expected to be most soluble?

A) CH2Cl2

B) H2O

C) Methanol

D) Ethanol

E) CCl4

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What is the strongest intermolecular force present in SO2 ? (EN Values: S = 2.5; O = 3.5)

A) Dispersion forces (London dispersion)

B) Dipole-Dipole force

C) Hydrogen-Bond

D) Ion-Dipole force

E) Covalent Bond

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Which of the following compounds should be soluble in CCI 4?

A. NaCl
B. C8H18
C. H2O
D. NH3
E. None of these compounds is soluble in CCI4

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Coca-Cola took a public relations hit a few months ago when it was discovered that their “scientists” stated that obesity was due to insufficient exercise and not sugary soft drinks (New York Times, August 9, 2015).

Why is sugar so soluble in water?

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Rank the following liquids from most soluble to least soluble in hexane (C 6H14)? Pentane (C5H12), Ammonia (NH3), Chloroform (CHCl3

A. pentane, pentane, ammonia
B. chloroform, pentane, ammonia
C. pentane, ammonia, chloroform
D. chloroform, ammonia, pentane

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Place the following compounds in order of decreasing strength of intermolecular forces.
HF        H2       CO 2

A) HF > CO2 > H2

B) HF > H2 > CO2

C) H2 > CO2 > H

D) CO2 > HF > H 2

E) CO2 > H2 > HF

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Which has the smallest dipole-dipole forces?

A) CH3Cl

B) HBr

C) O2

D) NO

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Classify the molecule AsCl3.

1. polar molecule with polar bonds

2. polar molecule with nonpolar bonds

3. nonpolar molecule with nonpolar bonds

4. nonpolar molecule with polar bonds

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Consider four molecules

I) CHCl3
II) CH4
III) CH3Cl
IV) CCl4

Which of these exhibit permanent dipole-dipole interactions?

1. I, III, and IV only

2. III only

3. None of these

4. I only

5. I and III only

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In which of the following solvents would carbon disulfide, CS 2, be most soluble?

a. Water, H2O

b. Chloroform, CHCl3

c. ethylamine, CH3CH2NH2

d. Hexane, C6H14

e. Methanol, CH3OH

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Choose the compound below that contains at least one polar covalent bond, but is nonpolar.

A) SCl2

B) CF2Cl2

C) BCl3

D) GeH2Br2

E) All of the above are nonpolar and contain a polar covalent bond.

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Predict which of the following molecules is non-polar.

a) HBr

b) CF4

c) H2S

d) NI3

e) CHCl3

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Which statement best describes the polarity of CF 2I2

a) the molecule is nonpolar because the dipoles of the C—I bonds cancel each other

b) the molecule is nonpolar because the dipoles of the C—F bonds cancel each other

c) depending on the arrangement of the surrounding atoms, the molecule could be polar or nonpolar

d) the molecule is always polar

e) the molecule is always nonpolar

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Which molecule below is nonpolar? 

a) SF4

b) OF2

c) PH3

d) CBrCl3

e) XeCl4

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The molecules below are described as either polar or non-polar. Which one is incorrectly described?

(a) OCN - Polar

(b) CH2Cl2 - Non-polar

(c) CO2 - Non-polar

(d) NH3 - Polar

(e) SO2 - Polar

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Of the molecules drawn below, which is non-polar? If they are all polar, then do not circle a structure. 

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Which one of the following molecules is polar?

a) PBr5

b) CCl4

c) BrF5

d) XeF

e) XeF4

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Which of the following isomers of PF 2Cl3 are polar?

a) only I

b) only II

c) only III

d) II and III

e) I, II, and III

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Complete this sentence:  The PCl 5 molecule has

            A)      nonpolar bonds, and is a nonpolar molecule.

            B)      nonpolar bonds, but is a polar molecule.

            C)      polar bonds, and is a polar molecule.

            D)      coordinate covalent bonds

            E)      polar bonds, but is a nonpolar molecule.

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Consider CH4 and CF4. Electronegativities: C = 2.5, H = 2.1, F = 4.0. Which statement is  false?

a. Both are sp3 hybridized at carbon.

b. The bond angles in CF4 are smaller than those in CH 4.

c. The C-F bonds are more polar than the C-H bonds.

d. Both molecules are nonpolar.

e. The bond dipoles in CF4 are directed toward the fluorine, but those in CH 4 are directed toward the carbon atom.

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Which one of the following is a nonpolar molecule with polar covalent bonds?

a. NH3

b. H2Se

c. SOBr2 (S is central atom)

d. BeCl2

e. HCl

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Which of the following statements about polar molecules is  false?

a. A molecule with polar bonds can be nonpolar.

b. A molecule with polar bonds can be polar.

c. A molecule with only nonpolar bonds can be polar.

d. A molecule with 120° bond angles can be polar.

e. A molecule with 180° bond angles can be polar.

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Which of the following shows these molecules in order from the most polar to least polar?

a. CH4 > CF2Cl2 > CCl4 CCl2H2

b. CH4 > CF2H2 > CF2Cl2 > CCl4 > CCl2H2

c. CF2Cl2 > CF2H2 > CCl2H2 > CH4 = CCl4

d. CF2H2 > CCl2H2 > CF2Cl2 > CH4 = CCl4

e. CF2Cl2 > CF2H2 > CCl4 > CCl2H2 > CH4

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Which statement best describes the polarity of CF 2l2?

a. the molecule is nonpolar because the dipoles of the C-l bonds cancel each other.

b. the molecule is nonpolar because the dipoles of the C-F bonds cancel each other

c. depending on the arrangement of the surrounding atoms, the molecule could be polar or nonpolar

d. the molecule is always polar

e. the molecule is always nonpolar

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Which of the following statements about SF5 is true?

1. The ion has polar bonds and the ion is polar.

2. The ion has polar bonds but the ion is nonpolar. 

3. The ion has nonpolar bonds and the ion is nonpolar. 

4. The ion has nonpolar bonds but the ion is polar. 

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Which of the following statements about SO 32- is true?

1. The bonds are polar and the ion is polar.

2. The bonds are polar and the ion is nonpolar.

3. The bonds are nonpolar and the ion is polar.

4. The bonds are nonpolar and the ion is nonpolar.

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Which of the following has bonds that are the most polar?

1. SO32–

2. PCl4+

3. IF5

4. SCl2

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Which of the following species is nonpolar?

1. SCl2

2. ClO3-

3. IF2-

4. IF5

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Which of these is nonpolar?

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Which of the following statements concerning polar molecules is false?

a. There must be at least one polar bond or one unshared pair of electrons on the central atom.

b. If there are more than one polar bond, they must not be symmetrically arranged so that their polarities cancel.

c. If there are more than one unshared pair of electrons on the central atom, they must not be symmetrically arranged so that their polarities cancel.

d. There must be an odd number of polar bonds so that their polarities not cancel.

e. A molecule with symmetrically arranged polar bonds can be polar if the central atom is bonded to atoms of different elements.

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