Formal Charge: Lewis dot structures

$FC=V-(L+\frac{S}{2})$

Jocelyn Fermin1J
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Joined: Tue Nov 14, 2017 3:01 am

Formal Charge: Lewis dot structures

Can someone explain how to find the formal charge for elements in Lewis dot structures? If you can include an example that would be very helpful

Andrea- 3J
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Joined: Tue Nov 14, 2017 3:00 am

Re: Formal Charge: Lewis dot structures

There's an equation to find formal charge.
FC=V-(N+(B/2))

FC=formal charge
V=number of Valence electrons
N=number of non-bonding valence electrons
B=total number of electrons in covalent bonds with other atoms in the molecule

Example: the formal charge of Oxygen (O) in Carbon Dioxide (CO2)
V=6
N=4
B=(4/2)=2

FC=6-(4+2)
FC=6-6
FC=0
Oxygen has a formal charge of 0.

Elizabeth Parker 1K
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Re: Formal Charge: Lewis dot structures

Do we always need to find formal charge?

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Re: Formal Charge: Lewis dot structures

Most likely yes because finding the formal charge helps to draw the most stable Lewis structure of the molecule.

Jacob Samuels 1E
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Re: Formal Charge: Lewis dot structures

Ideally, all your formal charges would be between -1 and 1, and you should change the structure to allow for the maximum amount of FC=0 as possible.

EllenRenskoff-1C
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Re: Formal Charge: Lewis dot structures

I am confused about when we can add double bonds versus when we just add electrons to certain elements in the given compound to make the formal charge closer to zero. How would we know what the best way to do that is in order to draw the proper Lewis dot structure?

Alejandro Salazar 1D
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Joined: Fri Sep 29, 2017 7:05 am

Re: Formal Charge: Lewis dot structures

EllenRenskoff-1C wrote:I am confused about when we can add double bonds versus when we just add electrons to certain elements in the given compound to make the formal charge closer to zero. How would we know what the best way to do that is in order to draw the proper Lewis dot structure?

On the attached file, you can see I added a double bond between both carbon because on the structure on left, only used 10 electrons. Therefore, I still needed to add two electrons. Hydrogen are an exception from the octet rule but not carbon. Carbon needs a full octet, 8 electrons. With this double bond, both carbon share 2 electrons, making both an octet rule.

You add double or triple bonds to reach octet by sharing electrons.
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