Dissolving Molecules

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Dissolving Molecules Powered By Docstoc
					Atomic View of Dissolving Compounds
Polar Covalent Bonds
• Electrons are unevenly shared in a bond between two atoms

• Electronegativity
    • Ability to attract electrons in a covalent bond
    • Higher electronegative atom in bond builds up negative charge
    • Lower electronegative atom in bond builds up positive charge


                              
       electron poor   H O           electron rich
                                    Bonding

                                a) Even: covalent
  Bonding

Electrons may or         b) Uneven: polar covalent
may not be evenly
     shared

                    c) Completely transferred: ionic
Why does water dissolve ionic
substances?
                                    Na+
                          O

                          
              H                   H      
                                                Cl-
    Cl-


                                  
          electron poor   H O            electron rich
Electronegativity Trends
increase
                       increase
Like Dissolves Like
   Oil and water don’t mix
     Water is a polar molecule
     Gas is primarily hydrocarbons (C and H)
      • nonpolar molecules
     isopropanol is a polar molecule but part of
     the molecule is more nonpolar and the
     other part is more polar
Lewis Dot Structures for Elements
   Place valence electrons around an atom
          symbol, two on each side
              Ne has 8 electrons

              Lewis dot Structure:



                     Ne
Bonding Trends (many exceptions)
  • Atoms pair up so that each sees at least a full
  valence shell (2 e- for H and 8 e- for other s and p
  block atoms)

  •To make neutral covalent compounds, atoms make
  1 bond for each electron that it needs to see 8
         • C has 4 valence electrons
            • C needs 4 electrons to get to 8
            • C makes 4 bonds

         •N has 5 valence electrons
            • N needs 3 electrons to get to 8
            • N makes 3 bonds
Drawing Lewis Structures of
Covalent Compounds
1. Count total number of valence electrons from all atoms
   in formula
   (Charged Molecules: add electrons if negative charge,
       subtract electrons if positive charge)

2. Most molecules are symmetric so organize the atoms
   in a symmetric fashion

3. If there is only one of an atom, it is probably central
   atom

4. Hydrogens go on the outside

5. Can be more than one central atom
   (carbons often make bonds with themselves)
Drawing Lewis Structures of
Covalent Compounds
 6. Place electrons around each atom
    • Try to fill outer shells of each atom (2 or 8 e-)
    • Don’t use more electrons than you start with

 7. Each pair of electrons between atoms makes one bond

 8. If atoms have not obtained an octet (8 e-) and are not
    exceptions, make multiple bonds by reorganizing nonbonding
    electrons
    • ie. N=N (double bond)
    • ie. NN (triple bond)

 9. Check to make sure that you didn’t use too many or too few
    electrons
Exceptions to Octet Rule
• C, N, O, F follow octet rule with very few exceptions

• H only wants to share 2 electrons

•Beryllium is happy sharing 4 electrons
   •ie. BeH2

• Boron is happy sharing 6 electrons
   •ie. BF3

• Third row and below can share more than 8 electrons
because of access to d orbitals (usually only if necessary)
   •ie. SF6
Shapes of Molecules
    The structure around a given
   atom is determined principally
    by minimizing electron cloud
             repulsions.
 VSEPR Shapes of Molecules
Bonds make up one bonding electron cloud
   single bonds – one electron cloud
   double bonds – one electron cloud
   triple bonds – one electron cloud

Non-bonding (lone pairs) of electrons make up one non-
bonding electron cloud

Electron clouds stay as far apart from one another as
possible

Non-bonding (lone pairs) electron clouds repel more than
bonding electron clouds
  Determining VSEPR Shapes of
  Molecules
1. Draw Lewis Structure

2. Identify the central atom or atoms whose geometry is of
   interest (A) (minimum of 2 atoms attached to have a geometry)

3. Count the number of bonding electron clouds around the atom
   (X)

4. Count the number of non-bonding electron clouds around the
   atom (E)

5. Determine the total number of electron clouds surrounding an
   atom (bonding + non-bonding)
   Types of Molecular Geometry
total electron clouds   180
                                   Linear
        2
    (2 bonding clouds
    AX2)
                        120

            3
    (3 bonding clouds          Trigonal Planar
    AX3)
  Types of Molecular Geometry
total electron clouds
        3               < 120
(2 bonding clouds                  Bent
1 non-bonding cloud
AX2E)
       4
(4 bonding clouds           109
AX4)

                            Tetrahedral
  Types of Molecular Geometry
total electron clouds
                                Pyramidal
       4
(3 bonding clouds
1 non-bonding cloud
AX3E)
                         107

       4
(2 bonding clouds
2 non-bonding clouds            Bent
AX2E2)                  105
Types of Molecular Geometry
total electron clouds
         5                                    120
 (5 bonding clouds
 AX5)                          90
                                     trigonal bipyramidal

      5               90
(4 bonding clouds
 1 nonbonding cloud
AX4E )                  120
                                     Distorted tetrahedral
  Types of Molecular Geometry
total electron clouds   90
         5
 (3 bonding clouds
 2 nonbonding clouds
 AX3E2)                   T-shaped

      5
(2 bonding clouds        180
 3 nonbonding cloud
AX2E3 )                   Linear
  Types of Molecular Geometry
                           90
total electron clouds
         6
 (6 bonding clouds
 AX6)
                                 Octahedral
                         90
                        90
      6
(5 bonding clouds
 1 nonbonding cloud       90
AX5E1 )                  square pyramidal
  Types of Molecular Geometry
total electron clouds
         6
 (4 bonding clouds              90
 2 nonbonding clouds
 AX4E2)                    square planar
Determining Polar Molecules
• Does the molecule contain at least one polar bond?
   • NO – not polar
   • YES – may be polar, determine shape of molecule to
   determine polarity
•Do the polar bonds cancel each other out or add together to give
overall polarity to the molecule?
   • Cancel out – not polar, typically base 2, 3, or 4 shapes (linear,
   trigonal planar, and tetrahedral) equal pulls cancel out
   • Add together – Polar, typically sub-base shapes pulls don’t
   cancel or molecules with different atoms causing pulls
                           
                            O



                                      
                    H               H
Polar Molecule Examples
                                                1. C-F bonds are polar
    CF4                      F                  2. Base shape
                                                   a. Tetrahedral
                                 109           3. Pulls cancel each other
                F            C
                                      F            provided each pull is
                     F
                                                   the same
                                                4. Not polar

    PCl3                                  1. P-Cl bonds are polar
                                          2. Sub-base shape
           Cl            P                   a. Pyramidal
                                 Cl

                Cl       107             3. Pulls don’t cancel
                                          4. Polar
Resonance Structures
   When molecules have “equivalent” lewis
   structures
   Problem 8.50

				
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posted:8/28/2012
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