double and triple bonds

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Savannah Mance 4G
Posts: 107
Joined: Fri Aug 30, 2019 12:17 am

double and triple bonds

Postby Savannah Mance 4G » Mon Nov 18, 2019 7:33 pm

I'm still pretty confused on when to do double and triple bonds on a Lewis structure. For example in the compound AsF3, I'm not sure if it's supposed to have a double bond on two of the fluorines. I calculated formal charge and with the double bonds on two fluorines, I get a formal charge of 0 on arsenic and all of the fluorines. If I do single bonds I get a formal charge of 2+ on arsenic and 0 on fluorine. Also, the Lewis is supposed to have a total of 26 electrons and I only got 26 when I did two double bonds. So, doesn't it make more sense to have two double bonds on two fluorines?

A Raab 1K
Posts: 56
Joined: Sat Aug 24, 2019 12:16 am

Re: double and triple bonds

Postby A Raab 1K » Mon Nov 18, 2019 7:48 pm

AsF3 would not have any double bonds. Instead, there should be a single bond from As to F, with each F having three lone pairs and As having one lone pair. This adds up to 26 electrons and they all have a formal charge of 0, as As has 5 valence electrons, which the 3 bonds and 2 lone electrons satisfy.

Elizabeth Harty 1A
Posts: 125
Joined: Sat Jul 20, 2019 12:16 am

Re: double and triple bonds

Postby Elizabeth Harty 1A » Mon Nov 18, 2019 8:01 pm

The formal charges should be minimized but they also have to add up to the overall charge of the molecule.

Alexis Webb 2B
Posts: 124
Joined: Thu Jul 11, 2019 12:15 am

Re: double and triple bonds

Postby Alexis Webb 2B » Mon Nov 18, 2019 8:05 pm

halogens, like F, usually like to have 3 lone pairs and one single bond.

Savannah Mance 4G
Posts: 107
Joined: Fri Aug 30, 2019 12:17 am

Re: double and triple bonds

Postby Savannah Mance 4G » Mon Nov 18, 2019 8:52 pm

Why does arsenic have lone pairs? And how would you know it would have lone pairs being the central atom?

Pablo 1K
Posts: 118
Joined: Sat Feb 02, 2019 12:15 am

Re: double and triple bonds

Postby Pablo 1K » Mon Nov 18, 2019 9:44 pm

Savannah Mance 3B wrote:Why does arsenic have lone pairs? And how would you know it would have lone pairs being the central atom?


From practice what helps me is abiding b y the rules of electronegativity. Though if it helps play with the problem by filling up all bound atoms valence shells and if electrons are left over it is an indicator that there may be LP on central atom (these electrons have to go somewhere).

Natalie Nartz 4F
Posts: 50
Joined: Thu Jul 11, 2019 12:17 am

Re: double and triple bonds

Postby Natalie Nartz 4F » Tue Nov 19, 2019 5:16 pm

Make sure that your formal charges equal the charge of the molecule, however they should still be minimized. Electrons should be counted before drawing the molecule to help determine how many bonds/lone pairs you will have


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