Figuring out which atom to include formal charge

[Tony Chen 1F]

When we're drawing Lewis structures and deciding which atom should carry the negative formal charge based on the overall charge of the molecule, do we always put a negative formal charge to the atom with the higher electronegativity since they're able to hold on to the extra electrons better? Conversely, what is the general rule for deciding which atom in the molecule to assign a positive formal charge?

[Lawrence Javelo Disc 3B]

Typically, this is true, we do put negative formal charges on atoms with higher electronegativity, but it's not the best to think of it this way. Understand that the formal charge of an atom is equal to the number of valence electrons minus lone pair electrons (dots) minus number of bonds (lines). This will give you if it's positive or negative.

[205686400]

Hi! Yes, this is technically true but it would be better for you to understand how to actually calculate the formal charge. Remember this formula:

Total charge = # of valence electrons - lone pairs - (shared bonds/2)

In other words count up the number of dots and lines attached to the atom and subtract it from that atom's specified number of valence electrons.

[Hannah_Pon_1F]

When you are comparing different structures of the same molecule and you are comparing formal charges, the "most likely" structure (if applicable) will have the most electronegative atom with the negative formal charge (assuming that it is impractical or not possible for the atom to have a formal charge of 0), not a positive formal charge. However, in general formal charges on any atom 0 or close to 0 is favored.

[Bruce Ruff 2B]

If you find yourself having to assign nonzero formal charges to atoms in a neutral molecule, you should probably reevaluate whether you have drawn the correct lewis structure first before worrying about electronegativities. However, in situations where you have to assign a charge, then yes you'll want to assign the negative charge to the more electronegative atoms and the positive charge to the less electronegative atoms. Also, I have noticed that in almost every lewis structure covered in class/hw where we have to assign a negative formal charge, it goes on oxygen.

[Andrea_Perez_2F]

Hello! When drawing the best resonance and most stable structure it is true that we are trying to minimize formal charges and place them on the most electronegative atom. Though, this may not always be the case. It is best to calculate the formal charge of each atom present to see which needs a negative or positive charge.

[Emaad Sohail 3F]

Usually, that's the case, yes. However, each molecule is different, and figuring out formal charge relies on a number of factors.

[Chelsea Greene 1L]

I always try to make sure each atom has the lowest formal charge possible, and then I begin to think about it in terms of electronegativity.

[Brooklyn Burgess 3L]

More formal charges in an atom makes it less stable, therefore, if you find yourself having to put high formal charges, it may mean you need to create a double bond.

[Brooklyn Burgess 3L]

Additionally, changing how you distribute atoms based on resonance can help.