Why do we use formal charge?


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David Chibukhchian 2G
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Why do we use formal charge?

Postby David Chibukhchian 2G » Mon Nov 02, 2020 9:53 am

Hey guys, I just had a question about making Lewis structures. I know that we have to use formal charge in order to distinguish between possible structures of a compound and figure out which is the most probable one. However, I'm kind of confused as to why this is the case. What does formal charge have to do with the structure of the compound? I hope my question makes sense, I just want to understand why we use this method. Any explanation would be really helpful!

Chem_Mod
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Re: Why do we use formal charge?

Postby Chem_Mod » Mon Nov 02, 2020 10:10 am

The formal charge is the charge on an atom assuming that all electrons are equally shared in a covalent bond. Generally, an atom or molecule is most stable when it has a formal charge of zero. When negatively charged, it has "too many electrons" and will generally be less stable because it wants to give up the excess electrons to be neutral. The opposite is true for an atom with a positive formal charge. So this is important for determining the best lewis structures, because your best structure is going to minimize the amount of formal charges that are on your atoms.

David Chibukhchian 2G
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Re: Why do we use formal charge?

Postby David Chibukhchian 2G » Mon Nov 02, 2020 10:55 am

That makes sense, I didn't realize it was about maximizing the stability of the compound itself. I get now that a negative formal charge would mean the atom wants to get rid of excess electrons, while a positive charge means it wants to gain more. A formal charge of 0 would thus make it more stable. Thank you, that definitely helped!

anikamenon2H
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Re: Why do we use formal charge?

Postby anikamenon2H » Mon Nov 02, 2020 11:44 am

Formal charge is used to figure out which structure has the lowest energy and hence is the most stable. Many molecules have multiple resonance structures and formal charge is the means which scientists use to determine which structure occurs in nature aka the most stable one. The goal is to get the formal charge as closer to zero as possible. It gets more complicated, I am sure Dr. Lavelle will discuss the caveats, but there are also guidelines that indicate that certain atoms tend to have certain formal charges in stable states. So to recap really quick, formal charge is just meant to determine which resonance structure is most accurate/stable as often times you will have 2 to 3 resonance structures so it helps you narrow it down and guides you where to place double and triple bonds vs lone pairs.

Hope this helps!

Gabriel Nitro 1E
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Re: Why do we use formal charge?

Postby Gabriel Nitro 1E » Mon Nov 02, 2020 12:18 pm

Hi,

Whenever we are constructing Lewis Structures, often times we arrive at situations where there is more than one valid structure. Note, I am not referring to resonance, I am referring to a scenario of whether or not you should have all singles bonds or include a double bond here and there as with the sulfate ion (SO4^3-). Thus, in these situations, the "answer" is typically the "best structure."

In order to arrive at the "best structure," we tend to pick the diagram which has the formal charges closest to zero. This serves to explain why we may choose a Lewis Structure of the sulfate ion (SO4^3-) with more formal charges of 0 than another variant with formal charges of 1-.

Hope this helps! :)

Eileen Quach Dis 2A
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Re: Why do we use formal charge?

Postby Eileen Quach Dis 2A » Mon Nov 02, 2020 1:26 pm

We calculate the formal charge for each atom so that we can modify the structure in order to get a total formal charge for the molecule of zero, which is more stable, and therefore more preferable.

Eileen Quach Dis 2A
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Re: Why do we use formal charge?

Postby Eileen Quach Dis 2A » Mon Nov 02, 2020 1:26 pm

We calculate the formal charge for each atom so that we can modify the structure in order to get a total formal charge for the molecule of zero, which is more stable, and therefore more preferable.

alebenavides
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Re: Why do we use formal charge?

Postby alebenavides » Tue Nov 03, 2020 4:27 pm

you use formal charge when trying to figure out which structure has the lowest energy (meaning its the most stable.

Jaclyn Dang 3B
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Re: Why do we use formal charge?

Postby Jaclyn Dang 3B » Tue Nov 03, 2020 4:30 pm

I dont want to repeat what is said above, so I wanted to add onto information about formal charge, the formal charge of each atom should add up to the total charge of the molecule

Jason Knight - 1F
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Re: Why do we use formal charge?

Postby Jason Knight - 1F » Tue Nov 03, 2020 4:47 pm

We use formal charge to help identify the number of electrons in the labeled molecule. A positive charge indicates the molecule has a lower amount of electrons present, and a negative charge indicates the molecule has more electrons present in relation to the molecule with no charge.

clairehathaway 2J
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Re: Why do we use formal charge?

Postby clairehathaway 2J » Tue Nov 03, 2020 4:57 pm

Jaclyn Dang 3A wrote:I dont want to repeat what is said above, so I wanted to add onto information about formal charge, the formal charge of each atom should add up to the total charge of the molecule

So just to clarify, if the molecule has an overall charge then it will be most stable when it's total formal charge is equal to the overall charge and if the molecule has no overall charge then the molecule will be most stable when it's total formal charge is equal to zero? I hope that makes sense!

Mackenzie Stockton 2H
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Re: Why do we use formal charge?

Postby Mackenzie Stockton 2H » Sun Nov 08, 2020 11:47 am

Usually, we are looking to draw a lewis structure with a formal charge equal to zero, because this is the most stable form. Many times there are many different possible ways we can bond atoms in a molecule via a lewis structure, so considering formal charge is the way to figure out the "correct" lewis structure that the molecule actually takes in nature. Small formal charges make for more stable molecules, with lower energy.

rhettfarmer-3H
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Re: Why do we use formal charge?

Postby rhettfarmer-3H » Sun Nov 08, 2020 11:41 pm

Thank you for posting this after reading Chem mods discussion this is what I retain because I am also having a hard time understanding. That formal charge is due to the stability of the element/molecule. hence, the FC is to be lowered for the sake of stability. And formal charge gives certain characteristics such as how reactive it is based on electrons.

Jack Kettering 3D
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Re: Why do we use formal charge?

Postby Jack Kettering 3D » Sun Nov 08, 2020 11:42 pm

We use formal charge to determine the best Lewis for structure for a molecule as we are trying to create a structure with the best balance and stability which is measured through formal charge.

Fernanda Olvera 3D
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Re: Why do we use formal charge?

Postby Fernanda Olvera 3D » Sun Nov 08, 2020 11:51 pm

Formal charge = helps to find which resonance structure is the most stable ( with the lowest energy).

Try to get the formal charge as close to 0.


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