When to use formal charge or octet rule?

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

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Andrew Wang 1C
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When to use formal charge or octet rule?

When drawing Lewis structures, how do we know when to use the formal charge or when to use the octet rule for each element?

Abby Lam 3F
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Re: When to use formal charge or octet rule?

I'm not too sure, but I think you should always check your drawing with formal charge to find whatever's the most stable structure.

EnricoArambulo3H
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Re: When to use formal charge or octet rule?

I would use both to find the ideal Lewis structure, but just as a reminder, you can have an expanded octet with p-block elements that have an energy level of 3 or higher because the other electrons will be in the d-orbital state. I think Professor Lavelle will explore this concept further in the next lecture. I hope this helped!

Benjamin Chen 1H
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Re: When to use formal charge or octet rule?

First, you want to apply the octet rule on each atom.
Then, you want to check the formal charges of each atom.

Generally, you want to have a Lewis Dot structure that has the least amount of Formal Charges, and you want the more electronegative atoms to have the negative Formal Charges and the less electronegative atoms to have the positive Formal Charges if formal charges can't be avoided.

MHarrold_1E
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Re: When to use formal charge or octet rule?

We use formal charge to find the most stable structure of a molecule. And for the octet rule, every atom must have 8 electrons except H that only has two. Exceptions to the octet rule occur with elements from period 3 and below. Those elements are allowed to have expanded octets because the d-shell is accessible for bonding. Hope this helps!

Emmeline Phu 1G
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Re: When to use formal charge or octet rule?

Hi! When drawing lewis structures, always apply the octet rule. Most elements with a few exceptions are most stable when they have eight electrons. However, in circumstances such as sulfur where they can have an expanded octet, the general guide for electrons does not apply. Whenever, you are drawing a lewis structure, you should check the formal charge to make sure the structure you drew is the most stable out of all the possible resonance structures. Hope this helps! :)

Joseph_Armani_3K
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Re: When to use formal charge or octet rule?

Usually the octet rule has to be kept in mind when showing a Lewis structure and when calculating formal charge, but formal charge is used to see which Lewis structure is the most stable. Also, some elements (those of level n=3) can expand their octet to make their formal charge 0. Phosphorus can form a maximum of 5 bonds because 5 valence - 5 bonds = 0. Sulfur can form a maximum of 6 bonds because 6 valence - 6 bonds = 0.

reyvalui_3g
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Re: When to use formal charge or octet rule?

If the molecule you are drawing has multiple resonance structures it is good to use formal charge in order determine that you have drawn the right representation of the atom.

Carly_Lipschitz_3H
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Re: When to use formal charge or octet rule?

When drawing the Lewis Structure, we need to apply the octet rule for every atom. After completing this step, we calculate the formal charges of each atom using the equation: formal charge = valence e- - (lone pair electrons + shared electrons/2). The goal is to create a Lewis Structure that has as many formal charges equal to 0 as possible, so then you would manipulate your initial Lewis Dot Structure to achieve this.

Madison Muggeo 3H
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Re: When to use formal charge or octet rule?

I start with the octet rule and then use formal charge to check over my work, and if there are any charges that needed to be added I will add them. It is helpful to use both, but you should shart with the octet rule.

Izamary Marquez 2H
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Re: When to use formal charge or octet rule?

Madison Muggeo 1G wrote:I start with the octet rule and then use formal charge to check over my work, and if there are any charges that needed to be added I will add them. It is helpful to use both, but you should start with the octet rule.

I do this as well to make sure I don't leave out any electrons! However, I was unsure... Have we learned specific units for formal charge? I never know what to put :/

Madison Muggeo 3H
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Re: When to use formal charge or octet rule?

Izamary Marquez 2L wrote:
Madison Muggeo 1G wrote:I start with the octet rule and then use formal charge to check over my work, and if there are any charges that needed to be added I will add them. It is helpful to use both, but you should start with the octet rule.

I do this as well to make sure I don't leave out any electrons! However, I was unsure... Have we learned specific units for the formal charges? I never know what to put :/

I don't think we ever learned units, so I would assume we would not need to put anything :)

Julianna_flores3E
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Re: When to use formal charge or octet rule?

I believe that you have to do both filling the octet and simultaneously figure out the formal charge in order to see the characteristic of the bonds.

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