Molecular Geometry:  Getting Bent Out of Shape Again!

Let's consider the bond angles in a variety of structures. The buttons () to the lower right of each Chime structure will allow you, after pushing each, to click on three consecutive connected atoms to measure the bond angle.  Results show up on the status bar of your browser window (lower left corner).  Measure the H-N-H bond angle in each structure given below. 

NH₄⁺                                      NH₃                                        NH₂^-

_______                            _______                            _______

What is the cause of the change in the H-N-H bond angle in the above structures?  You may want to draw the Lewis dot structures.

This is the typical distortion due to the repulsive nature of the lone pairs of electrons.  This was investigated in When Lone Pairs Rule.

Now let's examine the following structures.  Predict the H-C-H bond angle in each structure. 

CH₄                                    CH₃I                                    H₂CO

_______                            _______                            _______

_______                            _______                            _______

C₂H₆                              C₂H₄                              CH₃CN

_______                            _______                            _______

_______                            _______                            _______

Now measure the H-C-H bond angle in each compound.  

What is the cause of distortion in this series of structures?

Predict the O-N-O bond angle in NO₂ and then measure it.  Is this difference what you expected??

                Predicted  ________        Measured  _________

What is the cause of the distortion in this substance?

Any additional electron density (multiple bonds, large atoms such as iodine, or large groups such as methyl groups) cause distortion.  While in NO₂ the appearance of a lone (single) electron lessens distortion.

Under the right conditions in the gas phase a hydrogen atom can be removed from methane to form the methyl radical, ·CH₃.  No ions are formed.

CH₄           H·    +    ·CH₃    

Estimate the H-C-H bond angle in the methyl radical.  Why do you predict this value?  To see what really happens - Click here.

Compare the C-N-C bond angle in the structure of trimethylamine given below to ammonia (in the first set of structures).  Explain any difference.

Now consider the H-C-H bond angle in the structures of the cycloalkanes given below.  You may want to examine the C-C-C bond angle.  Explain any differences.

cyclohexane                             cyclobutane

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Scott A. Sinex    Prince George's Community College    8/2003