Molecular Geometry:  When Lone Pair Electrons Rule

How are the ideal geometries influenced by the lone pair electrons on a molecule?

What are the ideal bond angles in the molecules below, which lack any lone pairs?

Trigonal planar     Tetrahedral  Trigonal Bipyramidal Octahedral
BF3 CH4  PCl5 SF6



Examine the following molecules and compare their bond angles with the ideal geometries.   

Derived from trigonal planar geometry:


  angle (degrees - select 3 atoms)
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Derived from tetrahedral geometry:

NH3                                         H2



Derived from trigonal bipyramidal geometry:

SF4                                ClF3                                  XeF2



Derived from octahedral geometry:

IF5                                                 XeF4



How do the bond angles in the molecules compare to the values for the ideal geometries?

What do you conclude about the space occupied by a lone pair of electrons?  Why?

Do lone pairs always cause distortion in a molecule?  Explain.

Which is more repulsive - a bonded pair of electrons or a lone pair of electrons?  Explain.

For the trigonal bipyramidal geometry, the lone pairs of electrons are found in which position - axial or equatorial?  Why?

Are lone pair electrons the only cause of distortion?  View the two structures below and measure the H-C-H bond angle to assess this question.

Iodomethane                                 Formaldehyde


  <-- click on for space-filled model -->



Return to Gage CHM 101 webpage or Sinex CHM 101 webpage.

Scott A. Sinex        Prince George's Community College        10/2002