Shape of Molecules by ewghwehws

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									The Shapes of Things

  The Shape of Molecules
          Chapter Overview
• VSEPR Model – determining molecule
  structure
• Hybrid Orbitals – Incites into bonding and
  atomic structure
• Bond Length
• Polarity of molecules
Overview of VSEPR




     QuickTime movie
             VSEPR Model
• VSEPR Theory:
  – Valence Shell Electron Pair Repulsion

  – Minimizes the interaction between electron
    pairs around an atom

  – Allows us to approximate the structure of a
    molecule
  Predicting a VSEPR Structure
1. Draw the Lewis structure.
2. Put electron pairs as far apart as possible.
                      H
                      ••
                             Lone pair


                    •O•
                    •    •
                      ••
                      H
3. Determine the name of molecular
   structure from positions of the atoms.
           Orbital Geometry
                Linear
- There are 5 main types of orbital
  geometries (general shape of the molecule
  based on the number of groups, lone pairs
  + bonded atoms, off the central atom)




                   Linear
             2 groups off the central atom
Orbital Geometry
Trigonal Planar




Trigonal Planar
3 groups off the central atom
Orbital Geometry
  Tetrahedral




 Tetrahedral
4 groups off the central atom
   Orbital Geometry
 Trigonal Bipyramidal




Trigonal Bipyramidal
    5 groups off the central atom
Orbital Geometry
  Octahedral




  Octahedral
 6 groups off the central atom
       From Orbital Geometry
       to Molecular Geometry
- Once the orbital geometry has been
determined, the molecular geometry can
be determined.

-Molecular Geometry: general shape
of a molecule determined by the relative
positions of the atomic nuclei.
   - See the chart on the next page
 # of   Orbital              Molecular            # of lone pairs   Bond Angle
Groups Geometry              Geometry
   2   Linear                Linear                     0

  3   Trigonal Planar        Trigonal Planar            0

  3   Trigonal Planar        Bent                       1

  4   Tetrahedral            Tetrahedral                0

  4   Tetrahedral            Trigonal Pyramidal         1

  4   Tetrahedral            Bent                       2

  5   Trigonal Bipyramidal   Trigonal Bipyramidal       0

  5   Trigonal Bipyramidal   Seesaw                     1

  5   Trigonal Bipyramidal   T-shape                    2

  5   Trigonal Bipyramidal   Linear                     3

  6   Octahedral             Octahedral                 0

  6   Octahedral             Square Pyramidal           1

  6   Octahedral             Square Planar              2

            Lone pairs account for differences between the orbital and molecular geometries
                                                                                 Chart of Molecular Geometry
                                                                                      by: James Gorman
           Example – H2O

What does a water molecule look like?

- The molecular formula for water is H2O.
Draw the Lewis Structure.

                    •
                                H
                                ••

                    O         •O•
     H H
       •    •   •
                •
                     •
                         •
                         •
                              •
                                ••
                                   •


                                H
             Example – H2O
- Now, we examine the Lewis structure and
count how many lone electron pairs and
how many atoms we have bonded to the
central atom. These tell you how many
groups are off the central atom

      H
      ••
             Lone pair
                           2 Hydrogen atoms
    •O•
    •    •               + 2 lone pairs
      ••                  4 groups
      H
            Example – H2O
- Therefore the orbital geometry is
tetrahedral (4 groups).
- Since there are 2 lone electron pairs, the
molecular geometry is bent.

   H
   ••
 •O•
 •    •
   ••
   H
 Lewis         tetrahedral            bent
structure
          Practice in Groups
•   Take 15 minutes to determine the
    following molecular geometries:

a) NH3          b) CH4
c) O3           d) NO2-
e) H2CO         f) SiF4
g) BF3          h) SF6
         Practice in Groups
•   Take 15 minutes to determine the
    following molecular geometries:

a) NH3 – trig pyram.   b) CH4 - tetrahedral
c) O3 - linear         d) NO2- - bent
e) H2CO – trig pyram   f) SiF4 - tetrahedral
g) BF3 – trig planar   h) SF6 - octahedral
  Orbital Review From A
Covalent Bonding Perspective
          VSEPR Falls Short
• VSEPR is an excellent guide for predicting and
  understanding the shape of molecules. However,
  it does not provide much incite into the
  relationship between bonding and orbitals
  occupied by electrons.

• A new model is needed. . . hybridization

• Hybridization: mixing of two or more atomic
  orbitals of similar energies on the same atom to
  give new orbitals of equal energy
           Bond Length
• Distance between two bonded nuclei.
       Bond Length - Trends
• The periodic trend for bond length follows
  the trend for atomic radius.
  – F2 has a smaller bond length than Cl2 and so
    on

• Also, a single bond is longer than a double
  bond which is shorter than a triple bond.
  – More electrons between the nuclei allow them
    to come closer together (see table on next
    page)
Relation of Bond Order
    & Bond Length
         Molecular Polarity
• The polarity of a molecule can be derived
  from the electronegative difference
  between bonding atoms
• Looking at H2O, water, we have one type
  of bond (Hydrogen to Oxygen)
  EN of O = 3.5
  EN of H = 2.1
      (polar) 1.4
Intermolecular Forces Arising
From the Polarity of Molecules
• Dipole-dipole forces: attraction between
  two oppositely charged atoms on separate
  molecules
         Special Type of
    Dipole-Dipole Interaction
• Hydrogen Bonding: a special type of bond
  between hydrogen bonded to either a (F, O,
  N) and the unshared electron pair either (F,
  O, N). Some example are HF, H2O, NH3
       A Physical example of Polarity




An electrically
charged rod
attracts a stream
of chloroform but
has no effect on a
stream of carbon
tetrachloride.
London Dispersion Forces

								
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