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Chemistry 120

Lewis Structures & Shapes KEY KEY

Report Sheet

Lewis Structures & Shapes Part A: Follow the instructions in the procedure to complete this table. Formula

a. No. of valence electrons

H2O (example)

8

c. Shape/ Molecular geometry

d. Bond Angles

bent

109.5°

Yes

No

tetrahedral

109.5°°

Yes

No

H

trigonal pyramidal

109.5°°

Yes

No

H

bent/V-shaped

109.5°°

Yes

No

trigonal planar

120°°

Yes

No

b. Lewis Structure

e. Dipole Moment

H CH3Cl

14

H

C Cl H

NH3

N

8

H H

H2S

8

CH2O

12

S H O C H

H

1

f. Initials (if required)

Chemistry 120 Formula

a. No. of valence electrons

b. Lewis Structure O

SO42-

32

O

-

10

NO2-

18

NO2+

H

d. Bond Angles

tetrahedral

109.5°°

Yes

No

linear

180°°

Yes

No

bent/V-shaped

120°°

Yes

No

linear

180°°

Yes

No

linear

180°°

Yes

No

linear

N/A (no central atom)

Yes

No

2-

S O O

HCN

-

Lewis Structures & Shapes KEY f. Initials e. Dipole Moment (if required)

c. Shape/ Molecular geometry

-

-

C

N

-

N

-

O

O

16

O

N O

OCN-

16

O

N2

10

+

-

-

C

N

N N

2

Chemistry 120 Formula

a. No. of valence electrons

SO32-

26

H2Se

8

SCl2

20

PH3

8

d. Bond Angles

trigonal pyramidal

109.5°°

Yes

No

bent/V-shaped

109.5°°

Yes

No

bent/V-shaped

109.5°°

Yes

No

trigonal pyramidal

109.5°°

Yes

No

N

linear

180°°

Yes

No

Si F

tetrahedral

109.5°°

Yes

No

b. Lewis Structure

- S

O

Lewis Structures & Shapes KEY f. Initials e. Dipole Moment (if required)

c. Shape/ Molecular geometry

+

2-

-

- O O

Se H

H

S Cl

Cl P H

H H

SCN-

16

-

-

S

C

F SiF4

32

F

F

3

Chemistry 120 Formula

a. No. of valence electrons

OF2

20

F

CSe2

16

Se

32

-

d. Bond Angles

bent/V-shaped

109.5°°

Yes

No

linear

180°°

Yes

No

-

tetrahedral

109.5°°

Yes

No

I

trigonal pyramidal

109.5°°

Yes

No

H

tetrahedral

109.5°°

Yes

No

Cl

trigonal pyramidal

109.5°°

Yes

No

b. Lewis Structure

O F

C

Se

-

-

O ClO4-

O

Lewis Structures & Shapes KEY f. Initials e. Dipole Moment (if required)

c. Shape/ Molecular geometry

Cl O -

O

NI3

26

I

N I

+

H

NH4+

8

+

H N H

PHCl2

20

H

P Cl

4

Chemistry 120

Lewis Structures & Shapes KEY

Part B: Answer each question below. Refer to models if you find them helpful. 1. There are three different ways that you can make a molecule with formula C2H2Cl2. Draw all three. Below each, either draw an arrow indicating the direction of the dipole moment or write “no dipole moment” if there is none. In each case, carbon should make four bonds, while hydrogen and chlorine make one bond.

H

H C

Cl

Cl

C

H

C Cl

H

C

H

Cl C

Cl

H

C Cl

no dipole moment

2. Two students are asked to draw the Lewis structure for CH2Cl2 and state whether or not the molecule has a dipole or not. Their answers are shown below.

Both students’ Lewis structures are correct since it doesn’t matter which atoms are drawn in each direction. However, only one student is correct about the dipole. Does CH2Cl2 have a dipole? If so, clearly draw the dipole. Also, explain why one of the students was wrong, and describe how you could convince them of the correct answer. Answer with complete sentences. Student 1’s drawing implies that CH2Cl2 had no dipole moment since the most electronegative atoms are on opposite sides of the molecule. However, the molecule does indeed have a dipole moment. The two-dimensional Lewis Structures do not give an accurate representation of the molecule’s polarity. The three-dimensional structure shown below shows the dipole moment. By building a model it should be easy to show that the shape of the molecule is tetrahedral and that the polarity in the two C-Cl bonds do not cancel.

Cl H H

Cl 5

Chemistry 120

Lewis Structures & Shapes KEY

3. For each of the two polyatomic ions below, there is more than one way you can draw the structures by moving the positions of electrons in bonds and lone pairs. Each of these drawings is called a resonance structure for the molecule. Draw the indicated number of resonance structures for each ion. a. NO3- (nitrate ion – three resonance structures) Note that these three resonance structures should look like the same structure rotated 120°. We call these equivalent resonance structures to indicate that the bonding patterns are the same.

+ - N

O

-

-

O

-

-

O

+ - N

+

N

-

-

O

O

O

-

O O

O

b. OCN- (cyanate ion – two resonance structures, although more are possible)

O

C

+

N

-

O

+

C

+ -

N

Another resonance structure for cyanate ion is shown on the second page of the report sheet.

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