Formal charges not actual charges?

[Trayshawn]

"It is essential, however, to remember that formal charge is only a tool for assesing structures, not a measure of any actual charge on the atoms"

-from an inorganic chemistry textbook (Miesslerr)

What? This goes against everything I learned in organic. Formal charge destabilization is, for example, why a double bond with an -NH3 (and thus a nitrogen with a formal positive charge) on one side and would add the H on that side to avoid having the resulting carbocation be adjacent to the formal charge.
Or another easier to explain example - A positive formal charge is what makes water a good leaving group.

Point is, we utilize formal charges in more than just figuring out structure. But this inorganic book says otherwise and Im sure there is some rationale to it.

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[ClemsonFirst]

I think it means complete charge for actual charge as in ions while a formal charge is just a partial charge for different atoms in the molecule.

Because the electrons are shared, there is no actual positive charge due to an unbalanced electron to proton ratio (as in 7 protons in N while only having 5 electrons).

So, the nitrogen in NH3 still has access to the electrons because of its placement in the molecule. When electrons are transferred, as is the case in ionic compounds, the ions have the actual charge.

 

[Saint Fu]

I think it means complete charge for actual charge as in ions while a formal charge is just a partial charge for different atoms in the molecule.

Because the electrons are shared, there is no actual positive charge due to an unbalanced electron to proton ratio (as in 7 protons in N while only having 5 electrons).

So, the nitrogen in NH3 still has access to the electrons because of its placement in the molecule. When electrons are transferred, as is the case in ionic compounds, the ions have the actual charge.

To restate what (I think) he's saying, formal charges are basically an accounting tool to compare relative numbers of electrons and protons on an atom. It may not be the exact charge on that atom because of factors like electronegativity, geometry, conjugation, whatever, but in general it'll probably be pretty close.

 

[Geonsur]

Hello. I am in my first general chemistry course, and I am learning about formal charge for the first time. My book says that formal charge of any atom in a molecule is "the charge the atom would have if all the atoms in the molecule had the same electronegativity (that is, if each bonding electron pair in the molecule were shared equally between its two atoms).

Well, I understand that definition fine. The thing I don't get is: What's the purpose in finding formal charge? I don't get why we find it in the first place.

P.S. I wasn't really sure how to post a new thread about this, so I just clicked "Post Reply."

 

[ClemsonFirst]

They help you identify what the overall charge of a molecule is if you do not already know the charge. It also can be helpful in determining the proper resonance structure (the structure that contributes most to stability most often has the negative formal charge on the most electronegative atom).