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Molecular GeometryMolecular GeometryVSEPR - Valence Shell
Electron Pair Repulsion Theory.
In small molecules, electrons and bonds are arranged as far apart as possible.
Molecular GeometryMolecular GeometryThis allows for atoms to have 3-D shapes different from their shapes in 2-D (paper).
Molecular GeometryMolecular Geometry
Linear – the bonds form a straight line.
Example CO2
Bond Angle 180⁰
[http://cwx.prenhall.com/bookbind/pubbooks/hillchem3/medialib/media_portfolio/text_images/CH10/FG10_02-02d.JPG]
[http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter07/Text_Images/FG07_05-01UN.JPG]
Molecular GeometryMolecular GeometryTrigonal Planar–
The central atom has only 3 bonds.
Example: CH2O (Formaldehyde)
Bond angle 120⁰
[http://wps.prenhall.com/wps/media/objects/476/488316/Instructor_Resources/Chapter_10/FG10_00-66c.JPG]
[http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter07/Text_Images/FG07_05-01UN.JPG]
Molecular GeometryMolecular Geometry
Tetrahedral –The central atom has only 4 bonds
Example: CH4
(methane)Bond Angle
109.5⁰ [http://wps.prenhall.com/wps/media/objects/476/488316/
Instructor_Resources/Chapter_10/FG10_00-69d.JPG]
[http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter07/Text_Images/FG07_05-01UN.JPG]
Molecular GeometryMolecular GeometryTrigonal
Pyramidal –Central atom has 3 bonds and one lone pair of electrons that “pushes” the bonds away.
Example: NH3
(ammonia)
[http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter07/Text_Images/FG07_05-01UN.JPG]
Molecular GeometryMolecular Geometry
Bent – Here there are two bonds and two pairs of electrons on the central atom.
Example: H2O (water) [http://www.elmhurst.edu/~chm/vchembook/images/206water.gif]
[http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter07/Text_Images/FG07_05-01UN.JPG]
[http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter07/Text_Images/FG07_05-01UN.JPG]
PolarityPolarity
When 2 atoms in a bond have different electronegativity values, electrons can be “pulled” more toward one side of the bond.
A bond where this occurs is called a dipole.
[http://www.cybered.net/library/Teaching_Resources/Chemistry/Bonding_II/Image_Gallery/Bonding2--PolarBond.jpg]
[http://cwx.prenhall.com/bookbind/pubbooks/hillchem3/medialib/media_portfolio/text_images/CH09/FG09_11.JPG]
PolarityPolarityShowing Polarity“Minus” sign above
the high electronegative atom.
“Plus” sign over the other atom.
Draw arrow from plus to minus.
[http://cwx.prenhall.com/bookbind/pubbooks/hillchem3/medialib/media_portfolio/text_images/CH09/FG09_11-02.JPG]
Compound PolarityCompound Polarity
Draw a correct 2-D compound with arrows along each bond.
Example: CH4
CH
H
H
H
Compound PolarityCompound Polarity
Then, check to see if any of the arrows cancel out.
To do this think “tug of war”
CH
H
H
H
Compound PolarityCompound Polarity
If all arrows cancel out, the molecule is non-polar or covalent.
If the arrows do not cancel out, the molecule is polar or polar-covalent.