Lone Pairs

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

Cam Bear 2F
Posts: 60
Joined: Thu Jul 27, 2017 3:01 am

Lone Pairs

When drawing the molecular shape, how do you know where to put the lone pair(s)?

Stephanie tran 1J
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Joined: Fri Sep 29, 2017 7:04 am

Re: Lone Pairs

The first step to determining and drawing molecular shape is drawing the lowest energy lewis structure of the molecule to determine where the lone pairs go. Then you would go on to determine the shape of the molecule, the bond angles, and hybridization.

David Minasyan 1C
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Joined: Thu Jul 13, 2017 3:00 am

Re: Lone Pairs

It depends. For something like water, you'd put the lone pairs next to each other on top of oxygen to get that bent shape but for something like XeF4, then you'd be putting the lone pairs on opposite sides to get the right molecular geometry.

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Re: Lone Pairs

Essentially, you would follow the rules for a Lewis structure according to the Octet rule along with its exceptions. After you draw the molecule in a basic way, then you can evaluate where your lone pairs are to see if you need to redraw it. For example, for a seesaw shape, there would be four atoms attached to the central atom and a lone pair. If you drew the 4 atoms coming out of every side of the central atom, you would have to squeeze the lone pair in between two bonds. Thus, you can see from your initial drawing that you can adjust it and put the lone pairs by themselves on one side and two atoms coming out of the other side to form the seesaw. The electron pairs would have more repulsion power so you would put the lone pairs alone and the two atoms bonded to the other side opposite the lone pair.

Natalie LeRaybaud 1G
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Joined: Thu Jul 13, 2017 3:00 am

Re: Lone Pairs

In general you would put the lone pairs on the central atom, unless of course it follows the octet rule and can not have more than 8 valence electrons bonded. But overall pay attention to the octet rule and if the central atom follows it, then the lone pair is most likely to be placed on the central atom.

Kourtney Nham 1L
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Joined: Fri Sep 29, 2017 7:04 am

Re: Lone Pairs

Just to add on, in general, if there are multiple lone pairs, you want to put them as far apart as possible because they are extremely repulsive!