Lewis Concept
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Lewis Concept

The third conceptual definition of acids and bases is that of Lewis. You've heard of him before. Electron dot diagrams are sometimes called Lewis diagrams. Lewis focused his thoughts about acids and bases on what happens when a base accepts a proton and what structiral features of the base allow it to accept a proton. Let's look at what happens. (This is also shown in Ex. 23 in your workbook.) {Note: The charges shown below apply to the entire polyatomic ion.}

When a hydroxide ion accepts a proton, you can see that the hydrogen ion, the proton, attaches itself to a pair of electrons from the oxygen.
        ии  _
H : O :
    ии
+ H+ rtarrow.gif (850 bytes) ии
H : O : H
ии
When a hydrogen ion reacts with ammonia, it attaches itself to a pair of electrons on the nitrogen.
H
ии
H : N :   
ии
H
+ H+ rtarrow.gif (850 bytes) H
      ии   +
H : N : H
ии
H
When a hydrogen ion reacts with carbonate ion it attaches itself to a pair of electrons on one of the oxygen atoms that surround the carbon in the carbonate ion.
     ии          ии  2-
: O : C : O :
ии   : :   ии
: O :
+ H+ rtarrow.gif (850 bytes)   ии          ии    -
: O : C : O : H
ии   : :   ии  
: O :  

 

What happens in each case is that the base provides or donates a pair of electrons to the hydrogen ion in order to allow it to form a covalent bond. So, Lewis' definition of a base is that a base is an electron pair donor.

Now, think about what the hydrogen ion is doing in each case. The hydrogen ion is attaching to or accepting an electron pair. So, just as a base is an electron pair donor, an acid is an electron pair acceptor.

Once you say that an acid is an electron pair acceptor, then you're not limited to the idea that an acid has to be a hydrogen ion or some chemical that contains or releases hydrogen ions. It could be something else, as long as it can accept (or bond to) a pair of electrons.

One commonly cited example of a Lewis acid that does not contain hydrogen is boron trifluoride, shown here accepting a pair of electrons from ammonia. There are many other examples of Lewis acids. I won't go into them because we won't be dealing with them very much.

  ии
  : F :
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: F : B    
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  : F :
  ии
+ H  
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  : N : H
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rtarrow.gif (850 bytes) ии     
: F :   H
ии   ии     ии   
: F : B  : N : H
ии   ии    ии   
: F :   H
ии     
acid base

 

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