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02b - Drawing _ Naming Molecules Notes

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					  Drawing & Naming
  Molecules

       Covalent Bonding




Electron-Dot Diagram

 A representation of the number of valence
 electrons surrounding an atom.
                     • •
                     • •




  H•                • Br •
                         •
                                    •




                                 • N•
                                    •
          •
          •




         • O•
                                    •




            •
             •




Octet Rule

 An element with an octet of valence electrons
 has a stable electron configuration.

 The tendency of atoms to share electrons
 and form covalent bonds in order to achieve
 eight valence electrons is the octet rule




                                                 1
Types of Pairs

     In an electron-dot diagram, there are two
     kinds of electron pairs.
      shared pair or bonding pair
      unshared pair or lone pair or nonbonding pair
                        • •
                        • •

                    H • Br •
                      • •             unshared pair

shared pair




Lewis Structure

     A structural formula in which electrons are
     represented by dots; dot pairs or dashes
     between two atoms represent pairs of
     electrons in covalent bonds.




Writing a Lewis Structure

1.    Gather information
2.    Arrange the atoms
3.    Distribute the dots
4.    Draw the bonds
5.    Verify the structure




                                                      2
1. Gather Information

 Draw an electron-dot diagram for each atom
 in the structure. When placing valence
 electrons around an atom, pace one electron
 on each side before pairing any electrons.

 Determine the total number of electrons in
 the compound.




2. Arrange the Atoms

 Arrange the Lewis structure to show how the
 atoms bond in the molecule.
 Halogen and hydrogen atoms often bind to
 only one other atom and are usually at the
 end of the molecule.
 Carbon is often placed at the center of the
 molecule.
 The atom with the lowest eN is often the
 central atom.




3. Distribute the Dots

 Distribute electrons so that each atom
 satisfies the octet rule.
   Exceptions: hydrogen, beryllium, boron


 Always start with the central atom.




                                               3
4. Draw the Bonds

     Replaced the shared electrons pairs with a
     dash to indicate a bond.




5. Verify the Structure

     Count the number of valence electrons
     surrounding each atom. Ensure each atom
     (minus the exceptions) satisfies the octet
     rule.




Writing a Lewis Structure

1.    Gather information
2.    Arrange the atoms
3.    Distribute the dots
4.    Draw the bonds
5.    Verify the structure




                                                  4
Polyatomic Ions
 Follow the same steps for regular Lewis
 Structures but adjust the electrons.
 Draw brackets and the charge of the ions
 around the final structure.
                   •
                   •
                • O•            2-
                • |•    • •
                        • •
              • •
              • •


             • O —S — O•
             •         •
                  |
                •O•
                • •
                   •
                   •




Multiple Bonds

 Atoms can shared more than one pair of
 electrons.

 Double bond – a covalent bond in which two
 pairs of electrons are shared by two atoms.

 Triple bond – a covalent bond in which three
 pairs of electrons are shared by two atoms.




Resonance Structures

 Sometimes more than one Lewis Structure
 can be draw to represent a compound.

 These structures are known as resonance
 structures.
   E.g. O3




                                                5
Writing Covalent Compounds
 These compounds use prefixes to tell how
 many atoms of each element are present.

       mono-     1      hexa-     6


        di-      2      hepta-    7


        tri-     3      octa-     8


       tetra-    4      nona-     9


       penta-    5      deca-    10




Writing Covalent Compounds




                                            6

				
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