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Lewis Diagram

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					    SCH3U: Chemical
            Bonding

   Q: what did the bartender say
when oxygen, hydrogen, sulphur,
sodium and phosphorous walked
                  into the room?

                   A: OH SNaP!
Agenda
   Questions from Last Class?
   Ionic, Covalent and Metallic Bonding
   Lewis Diagram
       Single Atoms; hints for drawing Lewis diagrams
       Compounds
         Total Valence Electrons

         Steps to draw Lewis Diagram for compounds

   Practice Questions
   Reminder (Homework; Quiz; Gizmo)
Ionic bonding (Li + Cl)
    Ionic bonding (stealing/transfer of electrons)

     Li + Cl  [Li]+[Cl]–                   Chemical Formula

                                            Rutherford Diagram
            1e-
 3p+ 2e-1e-       7e- 8e- 2e-   17p+   3p+ 2e- 8e-8e-2e      17p+
 4n0                            18n0   4n0                   18n0




       Li             Cl                 [Li]+ [ Cl ]–
                                             Lewis Diagram
Ionic bonding (Mg + O)
   Mg + O  [Mg]2+[O]2–                    Chemical Formula

              1e-

12p+ 2e- 8e- 2e-    6e- 2e-   8p+   12p+ 2e- 8e- 8e- 2e- 8p+
12n0                          8n0   12n0                 8n0
               1e-
                                         Rutherford Diagram



      Mg             O                [Mg]2+ [    O     ]2–
                                            Lewis Diagram
Ionic bonding: Li + O
2Li + O [Li]2+[O]2– or 4Li + O2 2[Li]2+[ O]2–

                  1e-                 3p+ 2e-
     3p+ 2e-1e-
     4n0                              4n0
                                               8e- 2e-   8p+
                    6e- 2e-   8p+
                                                         8n0
                              8n0
     3p+ 2e-1e-                        3p+ 2e-
     4n0                               4n0
                  1e-


    Li                              [Li]2+ [     O       ]2–
                   O
    Li
Ionic vs. Covalent
Types of Covalent Bonds
   a. non Polar Covalent Bond: When EN
    difference is zero. Equal sharing of
    electrons.

   b. Polar Covalent Bond: When EN
    difference is >0 and <1.7. Results in unequal
    sharing of electrons, one atom has a higher
    affinity for the electrons.
Polar Covalent Bonds
Metallic Bond (neither ionic or
covalent)

   Occurs between two metal atoms.
   Valence electrons are released into a sea of
    electrons.
   Atoms release their electrons in a shared
    pool leads to stronger metallic bonds.
   Electrons are free to move and are not held
    together by a lattice formation (like that of
    ionic bonds
   That is why pure metals such as copper,
    sodium and gold are soft elements
Metallic Bonds
Chemical Bonding
       Lewis Structures
Lewis Diagram
   Single Atom: the chemical symbol and dots indicating
    the number of electrons in the valence energy level
   Compound: representation of covalent bonding based on
    Lewis symbols; Shared electrons are shown as lines and
    lone pairs as dots
To Write Lewis Diagrams
1. Write the element symbol. Around this, draw dots one for each
   valence electron.
2. The dots should be spread over four sides. Dots are not
   paired until all sides have at least one dot.
3. It does not matter on which side dots are placed. For example,
   hydrogen can be drawn four ways:


4. The number of valence electrons is equal to the group
   number. For example, hydrogen is in group IA (group 1) and it
   has one valence electron. Neon is in 0 (group 8) and it has 8
   valence electrons. The only exception is He which is in group
   8 but has 2 valence electrons.
Example: AsBr3
STEP 1:place the single atom
in the center and other atoms
around it evenly spaced


     Br As Br
        Br
STEP 2: count the total #
of valence e- for all atoms
involved in the bonding
Arsenic: 1 Arsenic with
        5 valence            AsBr3
        electrons (1x5) = 5
                              5+21
Bromine: 3 bromine             =26
            with 7 valence
            electrons (3x7) = 21
STEP 3: place the electrons
in pairs between the central
atom and each non-central
atom

         Br    As    Br

               Br
STEP 4: place the remaining
electrons around the non-central
atom until each has 8 electrons
(H atoms have only 2e-)
Ex: AsBr3

5 + (7x3) = 26
26 - 6 = 20      Br   As   Br

                      Br
STEP 5: if electrons remain
they are placed in pairs around
the central atom
 Ex: AsBr3        5 + (7x3) = 26
                  20 - 18 = 2


 Br    As    Br         Br As            Br
       Br                      Br
                            Structural
Exception: if the central atom is in
group 14, 15, 16, 17 or 18, the
octet rule must be satisfied by moving
electron pairs from non-central
atoms, creating multiple bonds.

 Ex: SO2
 6 +(6x2)
                    O S O
    =18
                   O- S = O
                            Structural Formula
  Resonance Structure
   When several structures with different electron distributions among
   the bonds are possible, all the structures contribute to the electronic
   structure of the molecule. These structures are called resonance
   structures
  EX) CO3




Whenconsidering the resonance structure…
  Formal charge closest to zero is favoured

  Negative formal charges on the more negative elements are favoured
Summary
   STEP 1:place the single atom in the center and other atoms around
    it evenly spaced
   STEP 2: count the total # of valence e- for all atoms involved in the
    bonding
   STEP 3: place the electrons in pairs between the central atom and
    each non-central atom
   STEP 4: place the remaining electrons around the non-central atom
    until each has 8 electrons (H atoms have only 2e-)
   STEP 5: if electrons remain they are placed in pairs around the
    central atom
   Exception: if the central atom is in group 14, 15, 16, 17 or 18, the
    octet rule must be satisfied by moving electron pairs from non-
    central atoms, creating multiple bonds
Summary / Hints
   When drawing Lewis Diagrams…
       Symmetrical arrangements are favoured
       Hydrogen is never a central atom
       The least electronegative element is usually the
        central atom
   When considering the resonance structure…
       Formal charge closest to zero is favoured
       Negative formal charges on the more negative
        elements are favoured
Practice Questions
        Draw Lewis Structure/ Structural Formula
         for the following compounds
    1.    CCl4
    2.    HCl
    3.    NH3
    4.    H2O
    5.    OH
    6.    H2
Reminder
   Do the practice problems and assigned
    homework
   Gizmo activity is due this Thursday!

				
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posted:8/29/2012
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