Molecular%20Geometry%20VSEPR%20Theory

1
Molecular Geometry VSEPR Theory

• Section 12.3

2
VSEPR

• Valence Shell Electron Pair Repulsion
• Electron pairs around central atom arrange
  themselves to be far apart as geometrically
  possible.
• Good help sites http//www.chem.purdue.edu/gchelp
  /vsepr/
• http//www.classzone.com/cz/books/woc_07/resources
  /htmls/ani_chem/chem_flash/popup.html?layeractsr
  cqtiwf_act065.1.xml

3
Linear

• 2 things
• Bond Angle 180o
• Ex CO2

180o
4
Trigonal Planar

• 3 things
• Bond Angle 120 ?
• Ex BCl3

120o
5
Tetrahedral

• 4 things
• Bond Angle 109.5 ?
• Ex CH4

109.5o
6
Unshared e-

• Unshared pairs of electrons are extremely close
  to the central atom compared to atoms forming
  bonds
• All the things are in the same orientation
  around central atom, but the only things we see
  in the molecule are the atoms forming bonds

7
Unshared e- (continued)

• For example NH3
• Draw the electron dot structure
• Determine how many things and also the shape
  based on things?

8
Unshared e- (continued)

• Therefore with NH3 we expect to see this because
  it has four things
• But one of these spheres would be replaced by a
  pair of e- REALLY close to the central atom, so
  we actually see this

9
Trigonal bipyramidal

• 5 things trigonal bipyramidal
• Bond angles (90?, 120?, 180?)
• Ex. SF5

10
Octahedral

• 6 things
• Bond angles (90 ?, 180?)
• Ex.PF6

11
Steps

• Draw electron dot or Lewis structure for
  molecule/ion.
• Count things around central atom.
• bonds (single, double, or triple each just count
  as one)
• non-bonding pairs (aka unshared pairs, lone
  pairs)
• These things are more technically called
  regions of electron density

12
Steps cont.

• Name the arrangement (5 possibilities)
• Things Arrangement
• 2 linear
• 3 trigonal planar
• 4 tetrahedral
• 5 trigonal bipyramidal
• 6 octahedral

13
Steps cont.

• a. If the central atom has no unshared pairs,
  this is the name of the geometry.
• Ex CH4 has 4 things so its tetrahedral
• b. If the central atom has unshared pairs,
  arrange them as far apart as possible.
• Doesnt matter for 2, 3, or 4 things because of
  symmetry
• When 5 things, lone pairs go on equatorial
  positions first (since they are farther away from
  other stuff)
• When 6 things, lone pairs go across from each
  other.
• Name the molecular geometry based on the bonds.