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Chapter 9_ Chemical Bonding

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Chapter 9_ Chemical Bonding Powered By Docstoc
					 Chapter 9: Models of Chemical
            Bonding
1.   Lewis Dot Symbols
2.   Ionic Bonding
3.   Metallic Bonding
4.   Covalent Bonding
5.   Polar Bonds
6.   Electronegativity
• Lewis symbols




    Examples : Na  and : C :

               Practice

• Give Lewis dot symbols for:
 magnesium            Oxide ion

 nitrogen             Sodium ion

 fluorine

 argon

 boron
Types of Bonding and Properties
             Ionic



            Metallic




            Covalent
               Ionic Bonding
• Ionic compounds form



• The attraction


• Forming ions takes energy
          Metallic Bonding
• Outer electrons

• Large numbers of metal atoms

• The electrons are ‘delocalized’
Covalent Bonds
         Network Covalent Solids


http://www.hull.ac.uk/chemistry/intro_inorganic/images/diamond.jpg
      Polar Covalent Bonds

• When two identical atoms form a covalent
  bond



• When different kinds of atoms combine,
(a)




(b)
• Electronegativity




• In general, electronegativity increases
http://www.chem.ufl.edu/~itl/2045/change/C9F16.GIF
• The difference in electronegativity



• There is no sharp dividing line between
  ionic and covalent bonding:

• A bond is mostly ionic when
   • The degree of polarity, or ionic
     character, varies continuously with the
     electronegativity difference
                               Each atom in a
                               bond has a partial
                               charge of about
                               +0.5 or –0.5 units
                               when the
                               electronegativity
                               difference is 1.7.

Electronegativity diff.   Type of bonding
         >1.7               Mostly ionic
       0.4-1.7             Polar covalent
         <0.4              Mostly covalent
          0               Nonpolar covalent
             Practice Questions
•    Identify the following bonds as
A.   Mostly ionic
B.   Polar Covalent
C.   Mostly Covalent
D.   Nonpolar Covalent

     C-Cl               P-H
     Re-H               Sn-Br
     Li-Cl              Sr-O
    • Lewis symbols can be used to
      represent the covalent or electron
      pair bond

          H  HH:H





 Formulas drawn with Lewis symbols are
called Lewis formulas or Lewis structures
•   The term structural formula


•   Many molecules obey the octet rule:



•   The number of bonds an atom (second-row)
    forms is determined by:
Number of Bonds
C   Has 4 e-   Needs 4 e-   Forms 4 bonds

N

O

F

B
• single bond

• Double and triple bonds

• The bond order

•   A single bond has bond order of
•   a double bond a bond order of
•   a triple bond a bond order of
•   Bond length depends on bond order:
    Drawing Lewis Structures
•   The least electronegative atom is usually in the
    middle. (Or the atom which can form the most
    bonds.)
•   Count total number of valence electrons in
    molecule/ion.
•   Place them around the atoms to satisfy the octet
    rule:
Phosphorus Trichloride




Carbon Dioxide
• Not all structures obey the octet rule
  Sulfur tetrafluoride




Bromine pentafluoride
• The preferred Lewis structure is the
  one that best fits the experimental
  data

                The structure of sulfuric acid in
                the vapor state. There are two
                different sulfur-oxygen bond
                lengths. The preferred Lewis
                structure needs different bond
                orders for these atoms.
                

              :O:                 :O:
              |              ||   

      H  O S  O H
               
                          H  O S  O H
                                   
                |                  ||
              :O:
               
                                  :O:
          StructureI         StructureII

• Structure I

• Structure II
• formal charge



• The formal charge on an atom is calculated
• Consider the sulfur atoms in the two structures
  for sulfuric acid:
                  

                 :O:                    :O:
                |                  ||   

        H  O S  O H
                 
                              H  O S  O H
                                       
                  |                      ||
                 :O:
                  
                                        :O:
            StructureI            StructureII
       Structure I: formal charge on S =
       Structure II: formal charge on S =
   When several Lewis structures are possible,



  
Nitrate ion




Iodate ion




Formal charge on O =   Formal charge on O =
Formal charge on I =   Formal charge on I =
• Some molecules and ions are not well
  represented by a single Lewis structure
• Consider the case of the formate ion, HCO2-




 Experiment gives


These are called resonance structures
  Draw the resonance forms for nitrate.




Draw resonance forms for thiosulfate.




Draw the resonance forms for dinitrogen monoxide.

				
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posted:3/31/2012
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