Trigonal Planar or Trigonal Pyramidal?

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

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Tyra Nguyen 4H
Posts: 74
Joined: Fri Sep 28, 2018 12:25 am

Trigonal Planar or Trigonal Pyramidal?

Postby Tyra Nguyen 4H » Sun Nov 25, 2018 2:21 pm

How do you know which shape is which? For example, SO32- has an electron arrangement that is tetrahedral, but its molecular shape is trigonal pyramidal. Why is it not trigonal planar?

Zenita Leang 2K
Posts: 67
Joined: Fri Sep 28, 2018 12:28 am

Re: Trigonal Planar or Trigonal Pyramidal?

Postby Zenita Leang 2K » Sun Nov 25, 2018 2:38 pm

Its molecular shape is trigonal pyramidal since it has a lone pair! Lone pairs will throw the electron shape off a little and thus SO32- has a bond angle of 109.5.

Keshav Bhatnagar 1H
Posts: 32
Joined: Fri Sep 28, 2018 12:17 am

Re: Trigonal Planar or Trigonal Pyramidal?

Postby Keshav Bhatnagar 1H » Sun Nov 25, 2018 3:17 pm

In order to find the electron - pair geometry, you need to count all electron densities including lone pairs. In order to find the molecular geometry, again you need to count all electron densities but especially be aware of how many lone pairs there are on the central atom.

This is because the molecular geometry is based on electron repulsion. For ex two lone pairs have a strong electron repulsion so the bond angle will be smaller thus forming a specefic shape.

LedaKnowles2E
Posts: 62
Joined: Fri Sep 28, 2018 12:27 am

Re: Trigonal Planar or Trigonal Pyramidal?

Postby LedaKnowles2E » Sun Nov 25, 2018 3:20 pm

If a molecule has a tetrahedral electron pair arrangement with one lone pair, the atom arrangement will be trigonal pyramidal because the lone pair takes up as much (and more) space than an atom in its place would. It repels the atoms into a trigonal pyramidal shape.

Aili Ye 4L
Posts: 58
Joined: Fri Sep 28, 2018 12:16 am

Re: Trigonal Planar or Trigonal Pyramidal?

Postby Aili Ye 4L » Sun Nov 25, 2018 3:36 pm

The structure would be trigonal planar if the lone pair did not exist. But the existence of the lone pair creates repulsion and takes up more space than an atom would and thus this would push the other molecules into a pyramidal shape.


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