Valence Electrons vs. Formal Charge when making Lewis Structures

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Yu Jin Kwon 3L
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Valence Electrons vs. Formal Charge when making Lewis Structures

Postby Yu Jin Kwon 3L » Sat Nov 14, 2020 10:44 pm

Hello!

Something that I have a bit of difficulty with is determining what the valence electrons or formal charges are in the lewis structures. For example, in the lewis structure below (which does not have the formal charges on the elements), when looking at Nitrogen, do you say it has 8 valence electrons (considering the bonds as two valence electrons each), or do you say it has 4 valence electrons (considering bonds as one valence electron each)? Then for formal charge, do you say it has a +1 charge (because 4 valence electrons when it should have 5 according to the periodic table I think)? I feel like my understanding of the connection between formal charge and valence structures when looking at lewis structures is pretty bad :0 Can someone explain which components of the lewis structure I have to look at to determine these differences?

Thank you!
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Thomas Vu 1A
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Re: Valence Electrons vs. Formal Charge when making Lewis Structures

Postby Thomas Vu 1A » Sat Nov 14, 2020 10:48 pm

So, I believe that due to the octet rule (with some exceptions in mind like H), all of the atoms should have 8 valence electrons under normal circumstances. As for formal charge, there is an equation to find it, but a shortcut that might be helpful is to just subtract the total number of bonds and single electrons from the number of valence electrons of the atom in the periodic table. hope that helps.

Shreya Patel- 2D
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Re: Valence Electrons vs. Formal Charge when making Lewis Structures

Postby Shreya Patel- 2D » Sat Nov 14, 2020 10:51 pm

The octet rule tells us that generally we must have 8 valence electrons. The formal charge for each atom can be found after drawing the lewis structure using the equation Formal charge= number of valence electrons needed - number of nonbinding electrons - (number of electrons shared in bonds /2). Hope that helps!

Stuti Pradhan 2J
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Re: Valence Electrons vs. Formal Charge when making Lewis Structures

Postby Stuti Pradhan 2J » Sat Nov 14, 2020 11:02 pm

The valence electrons just depend on the periodic table, so nitrogen would have 5. In the Lewis structure, you just want to make sure that nitrogen has a full octet. This means that it has 8 electrons, either as bonds or lone pairs. Each lone pair has two electrons and each bond has two electrons. The nitrogen atom in the image has 4 bonds, so therefore it has 8 electrons and a full octet.

Formal charge can be found by subtracting the number of bonds and the number of lone electrons from the original number of valence electrons. Since nitrogen has 5 valence electrons (as determined by the periodic table) and has 4 bonds in the diagram and no lone electrons, the formal charge is 5-4=1, so nitrogen has a formal charge of +1 in this situation. Similarly, oxygen has 6 valence electrons (again determined by the periodic table) and 3 bonds in this diagram as well as one lone pair (which means 2 lone electrons). To find the formal charge of oxygen, you can do 6-3-2=1. Oxygen here also has a +1 formal charge. Following the same process, carbon has a formal charge of -2 in the structure you have provided.

To answer your questions directly, you would say that nitrogen has 5 valence electrons, but has a complete octet in the Lewis structure. Your formal charge calculation for nitrogen is correct, and you can think about it in terms of electrons in the Lewis structure, but instead if you just count every bond as 1 and every lone electron as 1, the calculation might be easier. The technical equation that Dr. Lavelle gave us was
FC=V-(L+S/2), where FC is the formal charge, V is the number of valence electrons, L is the number of lone pair electrons, and S is the number of shared electrons (S is divided by 2 because each atom contributes 1 electron). This is basically what you were doing, but if it is ever confusing, going back to the original equation may help clarify some of the ideas, even if it is more cumbersome.

Hope this helps!

Yu Jin Kwon 3L
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Re: Valence Electrons vs. Formal Charge when making Lewis Structures

Postby Yu Jin Kwon 3L » Sat Nov 14, 2020 11:30 pm

Wow, thank you all so much!! These were fantastic explanations :))


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