Lesson 10
Home Up (15) Lewis Diagrams (16) Molecular Shape and Polarity (17) Intermolecular Bonding

 

(15) Lewis Diagrams

Obj. 15.  From the name of a molecular chemical, determine the Lewis (electron dot) diagram for it.

In these cases, you know that you are dealing with nonmetal atoms bonded together with covalent bonding and that (some of) the valence electrons of the atoms are shared between the atoms. What you need to figure out for each of these compounds is how those electrons are shared and depicted in terms of a Lewis diagram.

 

Exercises

For each of the following molecule compounds, draw the Lewis (electron dot) diagram for it.

a. hydrogen bromide

b. sulfur dioxide

c. sulfur trioxide

d. phosphorus trichloride

e. hydrogen sulfide

f. carbon monoxide

g. carbon disulfide

 

Worked-Out Examples

(a) Hydrogen bromide. This is a fairly simple case. Hydrogen has one valence electron and wants to gain one more; bromine has seven valence electrons and wants to gain one more. What will happen is that each of those atoms will provide one electron for the other. The electron dot diagram for that has the symbols for the two elements, H and Br with two dots between them representing the shared electrons. Bromine will also have six additional unshared dots for a total of eight.
H · ··
· Br :
··
   ··
H : Br :
   ··

 

(b) Sulfur dioxide. Sulfur dioxide, has a more complicated electron dot diagram. Presume that sulfur is in the middle because generally (but not always) the single atom in a molecular formula is the one that is usually in the center. Another generality is that the atom that needs the most electrons is in the center. In this case, both sulfur and oxygen, need two electrons so that doesn't give us any help. Another generality is that usually the most electronegative element is on the outside. That would also put sulfur in the center.

There are a number of ways to go about figuring out electron dot diagrams. This example shows the method that I generally use.

Start with the idea that sulfur is in the center.
O S O
It has six valence electrons represented by six dots. They don't have to be in any particular spots, but it makes sense to put two dots on the left, two dots on top and two dots on the right. The reason for that placement is that there is an oxygen atom on the left side of the sulfur and an oxygen atom on the right side of the sulfur. Each of those oxygen atoms wants to gain two electrons. So, put the dots on the sulfur so that each oxygen will be able to gain two electrons. The oxygen on the left side of the sulfur atom will gain the two electrons that are shown as dots on the left side of the sulfur. The oxygen on the right side will gain the two electrons on the right side of the sulfur. By careful placement of sulfur's electron dots we've kind of taken care of the oxygen in advance.
O ··
: S :
 O
Now what is the sulfur going to want? The sulfur wants two more electrons. The way this is usually done is to take two dots from either one of those oxygens. Let's use the one on the left. When we draw in the oxygen's six dots, put two of those dots on the right side of the oxygen so that they will be available to be shared with the sulfur.
·· 
O :
··  		··
: S :
 O
The sulfur no longer needs more electrons, so when dots are put around the second oxygen, the one on the right side of the sulfur, it will not have to provide any electrons to the sulfur.
·· 
O :
··  		··
: S :
 ·· 
O :
·· 
So here we have the electron dot diagram for SO2.

··     ··   ·· 
O : : S : O :
··          ·· 

 

Answers to Exercises

For each of the following molecule compounds, draw the Lewis (electron dot) diagram for it.

a. hydrogen bromide    ··
H : Br :
   ··
b. sulfur dioxide ··     ··   ·· 
O : : S : O :
··          ·· 
c. sulfur trioxide : O :
··   ::   ··
: O : S : O :
··        ··
d. phosphorus trichloride ··   ··   ··
: Cl : P : Cl :
··   ··   ··
: Cl :
··
e. hydrogen sulfide ··
H : S : H
··
f. carbon monoxide         
: C : : : O :
           
g. carbon disulfide ··            ··
S : : C : : S
··            ··

 

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E-mail instructor: Eden Francis

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