Trigonal Planar vs Trigonal Pyramidal

(Polar molecules, Non-polar molecules, etc.)

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kellyz_1C
Posts: 33
Joined: Fri Apr 06, 2018 11:05 am

Trigonal Planar vs Trigonal Pyramidal

Postby kellyz_1C » Tue Jun 05, 2018 1:48 am

How can you tell the difference between molecules that are trigonal planar vs trigonal pyramidal?

Bijal Luhar
Posts: 33
Joined: Fri Apr 06, 2018 11:03 am

Re: Trigonal Planar vs Trigonal Pyramidal

Postby Bijal Luhar » Tue Jun 05, 2018 3:07 am

Trigonal pyramidal has a lone pair at the central atom! While trigonal planar does not.

Alexandra Wade 1L
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Re: Trigonal Planar vs Trigonal Pyramidal

Postby Alexandra Wade 1L » Tue Jun 05, 2018 1:08 pm

The bond angles are also different for the two molecules. For trigonal planar, the bond angles are all 120 degrees while in trigonal pyramidal the bond angles would be 107 degrees because of the lone pair repressing the bonding pairs.

Hannah Lee 1B
Posts: 26
Joined: Fri Apr 06, 2018 11:03 am

Re: Trigonal Planar vs Trigonal Pyramidal

Postby Hannah Lee 1B » Tue Jun 05, 2018 2:58 pm

trigonal planar has 3 bonding regions and no lone pairs, with sp2 hybridization, while trigonal pyramidal has 3 bonding regions, 1 lone pair and sp3 hybridization. also in 3D trigonal planar would be like a flat triangle while trigonal pyramidal is pyramid-shaped

Luis Avalos 1D
Posts: 30
Joined: Wed Nov 22, 2017 3:00 am

Re: Trigonal Planar vs Trigonal Pyramidal

Postby Luis Avalos 1D » Tue Jun 05, 2018 6:06 pm

As people have stated, it has everything to do with lone pairs on the central atom

Alicia Beebe
Posts: 37
Joined: Fri Apr 06, 2018 11:01 am

Re: Trigonal Planar vs Trigonal Pyramidal

Postby Alicia Beebe » Wed Jun 06, 2018 12:11 am

The way I think of it is that trigonal planar has three regions of electron density, or three "attachments" to it. Trigonal pyramidal, however, has four. If you were looking at an atom with three bonds and a lone pair, you would see that it has four "attachments" which can give you the initial conclusion of tetrahedral. After reaching that structure, you then "take off" one of them (treating the lone pair as invisible) so the tetrahedral - 1 attachment = trigonal pyramidal.
This is because the lone pair electrons repel the bonded electrons, giving it a trigonal pyramidal shape. On the other hand, when there's no lone pair, they form the trigonal planar shape because they aren't being repelled by that lone pair.
This same process can be applied if there was an atom with two bonds and a lone pair. In this case, the "initial" idea would be trigonal planar, because of three "attachments", but if one was a lone pair, if you remove one from trigonal planar, you get angular.


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