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E-mail: shaefner@bridgew.edu
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Created 08/12/02
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Strategy for drawing Lewis Structures

1. Determine total # of valence electrons
   1. account for charge: each "+" subtract one electron; each "-" add one electron
      
      
2. Identify central atom
   1. least electronegative (some exceptions, e.g. H)
      
      
3. Draw skeletal form
   1. every atom needs to be bonded to another
      
      
4. Each bond is equivalent to 2 electrons
   1. subtract from total number
      
      
5. Use remaining electrons to complete octets
   1. fill terminal atoms first
   2. fill central atom last
      
      
6. If central atom lacks octet, convert l.p.'s on terminal atoms to bonding pairs (multiple bonds)

Exceptions to the Octet Rule

• Molecules with odd #'s of electrons
  
  
• Molecules with atoms having less than an octet
  • Be, B, Al
    
    
• Molecules with atoms that have an expanded octet
  • Atoms in periods 3 and higher (P, S, Cl, Br, I, Xe most common)
    
    
• Transition metals

Formal Charges

• A book keeping method useful for the assessment of Lewis Structures

formal charge of an atom

=

# of valence electrons in free element

-

number of lone pair electrons

-

number of bonds to the atom

• Charge on the molecule/ion must equal the sum of formal charges for each atom

 

Determining the relative importance of a Lewis Structure

("Not all Lewis Structures are created equal")

Rules for judging structures (in order of importance)

 

Resonance Hybrids

• The actual structure of a molecule is a weighted combination of all reasonable Lewis structures corresponding to given atomic arrangement
  • resonance is NOT a fluxuation or equilibrium between alternating structures
    
    
• Atom positions are the same but electrons are distributed differently
  => delocalization
  
  
• Resonance will average the bond characteristics (bond order, distance, charge) over the molecule
  
  
• Total energy of the resonance is lower than that of any one Lewis structure
  => stablization
  

Formal Charges on Specific Atoms

Formal charges may be quickly assigned by recognizing that particular combinations of bonds and lone pairs around a given atom will give specific formal charges. These are summarized in the table below:

Formal Charge	+1	0	-1
carbon
nitrogen
oxygen
halogen

Notes:

• replacement of a bond with a lone pair leads to a decrease in formal charge by 1
  
  
• bonds may be single bonds as shown, or multiple bonds e.g. -O- <=> =O