Exception to Octet Rule

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BKoh_2E
Posts: 92
Joined: Wed Sep 30, 2020 9:57 pm

Exception to Octet Rule

Postby BKoh_2E » Wed Nov 04, 2020 2:03 pm

In the Lewis structure for BF3 , why can't there be a double bond with one of the Fluorine atoms? Is formal charge always more important than having an octet?

Stuti Pradhan 2J
Posts: 155
Joined: Wed Sep 30, 2020 9:32 pm
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Re: Exception to Octet Rule

Postby Stuti Pradhan 2J » Wed Nov 04, 2020 2:11 pm

Formal charge just gives you the most stable structure for any molecule. In the case of BF3, if there was a double bond, fluorine would have a formal charge of +1. Fluorine is very electronegative and has a high electron affinity, so a +1 charge, which would indicate only 6 valence electrons instead of the original 7 from the parent element, or 8, which would be the preferred stable form, is very unstable. Since formal charge indicates how stable a structure is, it helps create a more accurate image of the structure of the molecule than simply following the octet rule, which is just an ideal and a guideline.

Hope this helps!

manisha_joseph_1H
Posts: 87
Joined: Wed Sep 30, 2020 10:01 pm

Re: Exception to Octet Rule

Postby manisha_joseph_1H » Wed Nov 04, 2020 2:11 pm

In his lecture, Professor Lavelle said that it would not be favorable for B to have a double bond with one of the F atoms because this would increase the formal charge of that F atom from 0 to 1. When more atoms have a formal charge of 0 in a Lewis structure, that molecule is more stable, which is what we want. Additionally, he mentions that F is the most electronegative element on the periodic table, meaning that it has a high electron affinity. As such, it would not be likely that an F atom would donate another electron to complete the octet of B as it wants to hold on to all of its electrons.


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