Structure and Bonding:  When Lone Pair Electrons Lose

Here are two similar looking molecules, NH3 on the left and NF3 on the right.  What ideal geometry are the two molecules derived from?

NH3                                                            NF3

   

Are the molecules polar or non-polar?  Explain.

Right click on each image and go to select, then to mouse click action, and click on angle. Measure the X-N-X bond angle, where X is H in ammonia and X is F in NF3.   

H-N-H bond angle  _________      F-N-F bond angle  __________

What can you conclude from the measurements?

We can examine the question above to a greater degree by converting the ball and stick model to an electrostatic potential surface.  The electrostatic potential surface is the distribution of charge on the molecule, where red is negative and blue is positive.  See the Student Guide for Using Chime for more explanation of the electrostatic potential surface and the menu steps- click here.  The colors are further shown on the figure below.

Source:  http://www.cmbi.kun.nl/wetche/organic/

What is the direction of the net dipole moment in each molecule?

Now examine the image of the NCl3 molecule given below.  What is the direction of the net dipole moment in this molecule?  If you produce the electrostatic potential surface, use the Rasmol potential color scheme and not the color red-white-blue.

Using the dipole moments given in the table below, justify these measurements with what you determined above using the geometry and electrostatic potential surfaces.

Compounds Difference in electronegativity in N-X bond Dipole moment (CRC Handbook, 1997) 
NH3   1.47 D
NCl3   0.39 D
NF3   0.29 D

Describe the direction of the dipole in each molecule.

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