Lone Pair Effect on Bond Angles  [ENDORSED]

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MichelleTran3I
Posts: 25
Joined: Fri Sep 29, 2017 7:06 am

Lone Pair Effect on Bond Angles

Postby MichelleTran3I » Mon Nov 27, 2017 10:38 am

How do lone pairs affect the standard bond angles for each shape? If a lone pair does exist, to what extent does it affect the bond angle and is there any way to calculate this?

Tim Foster 2A
Posts: 73
Joined: Fri Sep 29, 2017 7:07 am

Re: Lone Pair Effect on Bond Angles

Postby Tim Foster 2A » Mon Nov 27, 2017 11:10 am

Hey! Not sure about calculating the exact measure of bond angles being influenced by a lone pair, I believe that's a little above this course's pay grade, but I do know that bond angles in a molecule adjacent to the bond angle containing the lone pair will be slightly smaller than they would have been if the lone pair was another atom bonded to the central atom. For example, we have the molecule NF3, in which there is a lone pair on the nitrogen. The lone pair and the three fluorides give us four regions of electron density, which WOULD correspond to a tetrahedral structure and a bond angle of 109.5 IF AND ONLY IF all the regions of density repulsed each other equally. In reality, lone pairs have a greater repulsion power than fluorides bonded to nitrogen, or any atoms bonded to another atom, and so the bond angles here are slightly less than 109.5, which I believe is as accurate as we need for 14A. In this case the structure is trigonal pyramidal. Check out page 113 in the textbook, the illustrations are very helpful!

Kai_Chiu 1F
Posts: 19
Joined: Fri Sep 29, 2017 7:03 am

Re: Lone Pair Effect on Bond Angles

Postby Kai_Chiu 1F » Mon Nov 27, 2017 11:20 am

Tim Foster 3C wrote:Hey! Not sure about calculating the exact measure of bond angles being influenced by a lone pair, I believe that's a little above this course's pay grade, but I do know that bond angles in a molecule adjacent to the bond angle containing the lone pair will be slightly smaller than they would have been if the lone pair was another atom bonded to the central atom. For example, we have the molecule NF3, in which there is a lone pair on the nitrogen. The lone pair and the three fluorides give us four regions of electron density, which WOULD correspond to a tetrahedral structure and a bond angle of 109.5 IF AND ONLY IF all the regions of density repulsed each other equally. In reality, lone pairs have a greater repulsion power than fluorides bonded to nitrogen, or any atoms bonded to another atom, and so the bond angles here are slightly less than 109.5, which I believe is as accurate as we need for 14A. In this case the structure is trigonal pyramidal. Check out page 113 in the textbook, the illustrations are very helpful!


i agree with this, the bond structure would be trigonal pyramidal.

Chem_Mod
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Re: Lone Pair Effect on Bond Angles  [ENDORSED]

Postby Chem_Mod » Mon Nov 27, 2017 2:20 pm

The above posts are correct. To help you picture this visually, imagine the lone pair electrons "pushing" the other atoms away from them, or repelling the atoms. This would subsequently cause smaller bond angles between the other atoms.


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