Trigonal Pyramidal

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Ricky Ma DIS 4E
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Joined: Fri Sep 28, 2018 12:27 am

Trigonal Pyramidal

Postby Ricky Ma DIS 4E » Thu Nov 15, 2018 9:12 pm

Within my notes from lecture, it says it has 4 areas of electron density but i only see 3 for S03^2-. Is the double bond counting as two regions of electron density? Is that also why the arrangement is considered tetrahedral since of the double bond despite only having 3 oxygen molecules surrounding the sulfur thus making the bond angles slightly less than 109.5 degrees which is the number for tetra hedral. I got confused because trigonal planar has 120 degrees.

Brian Chhoy 4I
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Joined: Fri Sep 28, 2018 12:16 am

Re: Trigonal Pyramidal

Postby Brian Chhoy 4I » Thu Nov 15, 2018 9:16 pm

It has 4 regions of electron density because lone pairs count as areas of electron density as well. A double or triple bond is still considered only one region of electron density. The bond angles are less than 109.5 because lone pairs have a stronger repulsion than bonded pairs.

Ricky Ma DIS 4E
Posts: 72
Joined: Fri Sep 28, 2018 12:27 am

Re: Trigonal Pyramidal

Postby Ricky Ma DIS 4E » Thu Nov 15, 2018 9:17 pm

Wouldnt this make it not even close to a tetrahedral? why does it say in my notes that it has tetrahedral arrangement?

Ricky Ma DIS 4E
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Joined: Fri Sep 28, 2018 12:27 am

Re: Trigonal Pyramidal

Postby Ricky Ma DIS 4E » Thu Nov 15, 2018 9:19 pm

And in that case wouldn't it be slightly less than 120 degrees since trigonal planar is 120 degrees? SO3^2- is just 3 bonding regions with one lone pair.

Brian Chhoy 4I
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Re: Trigonal Pyramidal

Postby Brian Chhoy 4I » Thu Nov 15, 2018 9:20 pm

Its electronic shape would be that of a tetrahedral, while the molecular shape is that of trigonal pyramidal. Maybe you confused the two terms?

Ricky Ma DIS 4E
Posts: 72
Joined: Fri Sep 28, 2018 12:27 am

Re: Trigonal Pyramidal

Postby Ricky Ma DIS 4E » Thu Nov 15, 2018 9:22 pm

A trigonal planer has bond angles of 120 degrees though. 109.5 degrees is for tetrahedral species. so wouldnt the bond angles be slightly less than 120 degrees instead of 109.5?

Brian Chhoy 4I
Posts: 66
Joined: Fri Sep 28, 2018 12:16 am

Re: Trigonal Pyramidal

Postby Brian Chhoy 4I » Thu Nov 15, 2018 9:25 pm

The lone pair still plays a role in the shape of the molecule, thus making the electronic shape tetrahedral. Just picture a tetrahedral shape with the atom at the top missing, leaving a trigonal pyramidal shape, with bond angles that are slightly less than 109.5

Ricky Ma DIS 4E
Posts: 72
Joined: Fri Sep 28, 2018 12:27 am

Re: Trigonal Pyramidal

Postby Ricky Ma DIS 4E » Thu Nov 15, 2018 9:28 pm

ok thanks!

Brian Chhoy 4I
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Joined: Fri Sep 28, 2018 12:16 am

Re: Trigonal Pyramidal

Postby Brian Chhoy 4I » Thu Nov 15, 2018 9:29 pm

The shape of the molecule with the lone pair doesn't make the molecule trigonal planar. The lone pair is still there and thus keeps the electronic shape tetrahedral, and its molecular shape trigonal pyramidal.

Ahmet_Dikyurt_3L
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Joined: Fri Sep 28, 2018 12:23 am

Re: Trigonal Pyramidal

Postby Ahmet_Dikyurt_3L » Thu Nov 15, 2018 10:09 pm

Is it because the electrons are so small (er than atoms), they don't get to be part of the naming? Because I know that only atoms are used in the naming.

Daniel_Frees_1L
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Joined: Fri Sep 28, 2018 12:18 am

Re: Trigonal Pyramidal

Postby Daniel_Frees_1L » Thu Nov 15, 2018 10:24 pm

Just to add on to what Brian said, the reason why lone pairs cause more repulsion is because they are more diffuse than a bond between atoms.


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