PF3 and NH3 geometry

[wantVCUdental]

NH3 is tetrahedral, but PF3 is trigonal bipyrimidal... why is that?

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[hausee]

N is in VA so it has 5 Valence electron, since each H can only bond to one electron when forming NH3, there is one pair of electron left to form lone pair. So NH3 has 4 electron reigon, 3x N-H and one lone pair. The angle now is not exactly 109 though.

same thing to PF3, tetra for both

 

[porterhouse]

NH3 is tetrahedral, but PF3 is trigonal bipyrimidal... why is that?

Very important. Clarify which type of geometry the question is asking about before answering it. The electron domain geometry is tetrahedral for both but the molecular geometry is trigonal pyramidal (not trigonal bipyramidal for PF3) for both. Both geometrical assignments are due to the lone pair of electrons on the central atom(s) of attachment. Draw them out, assign bonds and electrons, and you'll find what you're looking for.

 

[hausee]

Yup, I actually missed that. Molecular geo doesn't count electron lone pair. see here

http://forums.studentdoctor.net/showthread.php?t=352545

Very important. Clarify which type of geometry the question is asking about before answering it. The electron domain geometry is tetrahedral for both but the molecular geometry is trigonal pyramidal (not trigonal bipyramidal for PF3) for both. Both geometrical assignments are due to the lone pair of electrons on the central atom(s) of attachment. Draw them out, assign bonds and electrons, and you'll find what you're looking for.

 

[wantVCUdental]

thank you very much for the clarification. Appreciated.

 

[porterhouse]

Yup, I actually missed that. Molecular geo doesn't count electron lone pair. see here

http://forums.studentdoctor.net/showthread.php?t=352545

I don't think you missed anything, in fact you were right to some degree. These are the types of questions you encounter on practice tests that you ultimately kick yourself for getting wrong, and are what may potentially lead to wrong answers on the DAT if you don't read what the question is asking very carefully. This is a great example in my opinion.