Dipole Moment

Dipole arrows are used anytime a molecule possesses a dipole moment, which happens when a molecule is polar. 

Electronegativity

Concept: Understanding Electronegativity

3m
Video Transcript

Before we begin to master how to draw a chemical structure, it's first important to know certain things about it. One of them, being the use of dipole arrows and dipole arrows are connected to an idea such as electronegativity. Now we already know what electron affinity is, electron affinity is the lacking of an element towards accepting electrons. Remember that non-metals like to gain electrons, so they have high electron affinity numbers. Metals on the other hand like to lose electrons, therefore their electron affinities are lower. Electron affinity is similar to electronegativity. It's just the liking of element has towards electrons. Now we're going to say, connected to electronegativity is the idea of polarity. And we're going to say polarity arises when 2 elements are connected to each other and there's a major difference in their electronegative numbers.
Here I provide you guys with the chart of all the electronegative numbers of a majority of the elements in the periodic table. Now of course you don't have to memorize every single one, but there are some that you should become familiar with. For example, Hydrogen here is 2.2, now depending on which edition of your book you're using, some of you may see 2.1 also. Just realize that 2.1, 2.2 they're very similar like they're only off by 0.1, so if you go with either number it's okay. Then we're going to say the other elements that you should become familiar with in terms of their electronegative numbers are Carbon to Fluorine, and then Fluorine down to Iodine. These are the ones that you should become familiar with. And I know it's a lot to memorize but just realize from Carbon to Fluorine, they're only off by 0.5 from each other. Fluorine is the most electronegative number on our periodic table, it's 4.0. And everything to the left of it would be 0.5 less. So Oxygen take off 0.5, you get 3.5. Nitrogen take off another 0.5 to get 3.0. Carbon take off another 0.5 to get 2.5. That's basically half of the numbers that you should memorize, and then the other ones just look for common trends, from going from Fluorine to Chlorine to Bromine to Iodine.
Now based on what we've seen here on this chart, we noticed that the trend is electronegativity increases going from left to right of the periodic table and it decreases going down a group. This makes sense because as we go from left to right, we become more like the non-metals. Non-metals want to be negative because they want to become like the noble gases, therefore they have high electronegative numbers. Metals on the other hand like to lose electrons, so they don't like electrons as much, so their electronegative numbers would be lower. And as we go down a group, we become more like the metals. 

Electronegativity measures how likely an element within a bond will attract electrons more or less to itself. 

Dipole Arrows

Concept: Understanding Dipole Arrows

1m
Video Transcript

We're going to say, to show the difference in electronegativity between 2 elements bonded together, this is when we use dipole arrows. We're going to say the dipole arrow always points towards the more electronegative element. So if we had 2 elements connected together, let's say Element A connected to Element B, and let’s say B was the more electronegative one, we just say that the dipole arrow points towards B, the more electronegative element. And we're going to say that this kind of looks like a positive here and it is on the side of the less electronegative element.  

If a polar bond exists then dipole arrows must be used. 

Concept: Dipole Arrows and Bond Classifications

3m
Video Transcript

We're going to say here the difference in electronegativities can have a big effect on the properties of bond and the properties of the compound. And we can classify them as different things. We're going to say if we have a connection between 2 elements, let's say we have a connection between Carbon and Hydrogen. Carbon's electronegative number is 2.5, Hydrogen's electronegative number is 2.2, their difference would be 0.3, and this is their ∆EN, their difference in electronegativity. We're going to say based on the value of the difference, we can classify the bond, and we can classify the compound as different types of compounds. So if we have an electronegative difference of 0 that means that it's classified as a pure covalent bond.
And a good Example of that would be Br connected to Br, so basically Br2. They both have the same electronegative numbers, so when you subtract them from each other it's 0, so we would call them a pure covalent compound. If the difference is small, between 0.1 and 0.4, then we call it a non-polar covalent compound or non-polar covalent bond.
Good Example we just saw with this Carbon and Hydrogen. When we subtract their differences, we get a number of 0.3. So a compound could be CH4. Intermediate, we call them polar covalent and they're usually between 0.4 and 1.7.
So let's say we had Hydrogen connected to Br, their difference would fit somewhere between 0.4 and 1.7. And let me remove myself from this image so we have more room. And then if the difference was greater than 1.7 then it will be classified as an ionic bond.
So if we had NaF, definitely their numbers would be greater than 1.7. So their bond will be classified as an ionic bond.
So just remember, electronegativity is similar to electron affinity. It just means the liking of an element towards electrons. We're going to say this connects to the idea of polarity and later on when we start to draw compounds, we'll see how it plays a role in the way we draw these compounds.

Depending on the difference in electronegativity, a chemical bond fits within a certain category. 

Dipole Arrow Calculations

Example: Based on each of the given bonds determine the direction of the dipole arrow and the polarity that may arise. 

a.  H –––– Cl

b.  S –––– O

c.  Br –––– B –––– Br

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Problem: Based on the given bond determine the direction of the dipole arrow and the polarity that may arise. 

 H-C


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Problem: Based on the given bond determine the direction of the dipole arrow and the polarity that may arise.

N-F

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Problem: Based on the given bond determine the direction of the dipole arrow and the polarity that may arise.

H-N-H

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Problem: Answer each of the following questions dealing with the following compounds.

a) Which of the following compound(s) contains a polar covalent bond?

b) Which of the following compound(s) contains a pure covalent bond?

c) Which of the following compound(s) contains a polar ionic bond?

d) Which of the following compound(s) contains both a polar ionic bond and a polar covalent bond?


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Dipole Moment Additional Practice Problems

If a molecule has a symmetrical arrangement of polar bonds, then that mlecule is

a. nonpolar

b. somewhat polar

c. reverse polar

d. highly polar 

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Classify the C-Cl bond in CCl 4 as ionic, polar covalent, or nonpolar covalent.

a. nonpolar covalent 

b. ionic 

c. polar covalent

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For each compound below, click in the box to toggle the direction of bond polarity (leave the nonpolar bond).

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Why don't all molecules with polar bonds have a net dipole? 

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Which of these bonds to carbon would you expect to be relatively reactive: C—H, C—C, C—I, C=O, C—Li? Explain.

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Choose the correct polarity of the bonds within SO2

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Of the molecules SiF4 and SiBr4, which has bonds that are more polar?

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Of the molecules AlCl3 and AlBr3, which has bonds that are more polar?

a. AlCl3
b. AlBr3

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Which of the following bonds would you expect to be the most polar?

a. P-S
b. C-N
c. O-F
d. N-O
e. C-O

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

a. SeBr4

b. HCN

c. ICl3

d. CF4

e. Both B and C are nonpolar and contain a polar covalent bond.

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Which one of the following bonds is the most polar?

a. H-Cl

b. H-S

c. H-F

d. H-P

e. H-O 

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Choose the bond below that is most polar.

a. H-I

b. H-Br

c. H-F

d. H-Cl

e. C-H

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Which molecule should have higher dipole moment, and why?

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Which of the following DOES NOT have a dipole moment?

a) CO

b) NCl3

c) HCl

d) SF4

e) BCl3

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Answer each of the following questions dealing with the following bonds.

N—O               K—F                B—N                S—Cl               Br—Br              C—H

a. Which of the following is expected to have the  greatest bond polarity?

 

b. Which of the following is expected to have the  lowest bond polarity? 

 

 

 

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The electrons in a bond are not always shared equally. For which bond below are the electrons shared most unequally?

1. H — F

2. H — I

3. H — Cl

4. H — Br

 

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Identify the compound with the most polar bond.

A. NH3

B. AsH3

C. SbH3

D. PH3

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Choose the bond below that is  least polar.

a. C-Cl

b. C-Br

c. C-F

d. C-l

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Arrange the following bonds in order of *increasing* ionic character

H—Cl

C—H

H—H 

O—H

A) C—H < H —Cl < O—H < H—H

B) C—H < O—H < H—Cl < H—H

C) H—H < C—H < O—H < H—Cl

D) H—Cl < C—H < H—H < O—H

E) H—H < C—H < H—Cl < O—H

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Place the following in order of decreasing dipole moment.

I. cis-CHCl=CHCl     II. trans-CHCl=CHCl     III. cis-CHF=CHF

A. II > III > I    

B) III > I > II    

C) I = III > II    

D) II > I > III    

E) I > III > II

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Which of the molecules listed has the most polar bonds between the central atom and the oxygen atom?

a) OF2

b) NO3–

c) SO3

d) CO32−

e) ClO4

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Arrange the following bonds in increasing order of bond polarity (placing the one with the lowest polarity first)

H-H, O-H, Cl-H, S-H and F-H

a.  H-H < O-H < Cl-H < F-H < S-H

b.  F-H < S-H < Cl-H < H-H < O-H

c.  S-H < O-H < F-H < Cl-H < H-H

d.  F-H <S-H < Cl-H <H-H < O-H

e.  H-H < S-H < Cl-H < O-H < F-H

Watch Solution

How many of the following are polar covalent bonds?
           C-Cl            C-S      N-H         H-O

a. 0              b. 1                  c. 2                 d. 3                   e. 4

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Place the following bonds in order of increasing polarity.

C—Cl    C—C    C—F    C—I

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Magnitude of Dipole moment is dependent on the difference in electronegativity.  

Arrange the following in order of increasing dipole moment:  Si—O, C—Br, Br—I.

 

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Which of the following bonds would be the most polar without being considered ionic?

a) Mg–O

b) C–O

c) O–O

d) Si–O

e) N–O

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Circle the bond that is the most polar.

a) I—Br

b) Rb—F

c) O—F

d) F—F

e) N—O

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Which species below contains a pure covalent bond?

a) Li2CO3

b) SCl6

c) Cl2

d) PF3

e) NaCl

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What type of bonding is found in the molecule HCl?

1. ionic

2. nonpolar covalent

3. polar covalent

4. metallic

 

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Given the electronegativities below, which single bond has the lowest degree of polarity?

i. Element:                      H      C      N      O

ii. Electronegativity        2.1    2.5    3.0    3.5

a. C-H

b. N-H

c. O-H

d. O-C

e. O-N

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Which molecule below has the largest dipole moment?

a. HBr

b. HI

c. HF

d. HCl

e. H2

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Determine the answer for the question below.

Which of the following substances has polar covalent bonds?

1. SO2

2. Na2S

3. Cl2

4. LiCl

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In which case is the bond polarity incorrect?

    δ+  δ-             δ  +  δ-               δ+  δ-               δ+  δ-

1. S     Cl          2. C     F          3. O     S          4. H     O

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Choose the bond below that is least polar.

a. C–Cl

b. C–Br

c. C–F

d. C–I

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Place the following in order of increasing dipole moment.

I. BCl3 

II. BBrF

III. BClF2

 

A) I < II = III

B) II < III < I

C) I < II < III

D) II < I < III

E) I < III < II

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Place the following in order of decreasing dipole moment.

I. cis-CHCl=CHCl

II. trans-CHCl=CHCI

III. cis-CHF=CHF

 

A) III > I > II

B) II > I > III

C) I > III > II

D) II > III > I

E) I = III > II

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Which of the following species has bonds with the most ionic character?

1. SnO2

2. P4O10

3. CO2

4. PCl3

5. NO2

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Which of the following has the bonds ranked in order of increasing ionic character?

1. Cl-Cl < Si-Cl < Br-Cl < Mg-Br < Rb-Cl

2. Mg-Br < Rb-Cl < Si-Cl < Br-Cl < Cl-Cl

3. Cl-Cl < Br-Cl < Si-Cl < Rb-Cl < Mg-Br

4. Cl-Cl < Br-Cl < Si-Cl < Mg-Br < Rb-Cl

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Which of the following substances has polar covalent bonds?

a. SO2

b. Na2S

c. Cl2

d. LiCl

Watch Solution