Why exactly do we need formal charge? [closed]








**Closed.** This question is [off-topic](/help/closed-questions). It is not currently accepting answers.
 
 






---








 **Homework questions** must demonstrate some effort to understand the underlying concepts. For help asking a good homework question, see: [How do I ask homework questions on Chemistry Stack Exchange?](https://chemistry.meta.stackexchange.com/questions/141/how-do-i-ask-homework-questions-on-chemistry-stack-exchange)




Closed 2 years ago.








[Improve this question](/posts/157660/edit) 



Formal charge is considered to be the charge present in one atom by considering all the bonds to be 100% covalent.What is the significance of 100% covalent here? Why are we considering the bonds to be covalent and what is formal charged used for ?





> 
> Formal charge is considered to be the charge present in one atom by considering all the bonds to be 100% covalent.
> 
> 
> 


The "charge present in one atom" is not a clear concept. A better way is to say "formal charge is the charge assigned to an atom symbol in a Lewis structure". This acknowledges that the formal charge depends on the choice of Lewis structure. For some structures, you can write multiple plausible Lewis structures, which might show distinct formal charges.



> 
> Why exactly do we need formal charge?
> 
> 
> 


We don't. It is useful in deciding which Lewis structures make sense, and to get a rough sense of the charge distribution (and the net charge). There are better but more difficult methods to get at the actual atom-by-atom charge distribution , e.g. <https://en.wikipedia.org/wiki/Voronoi_deformation_density>. However, the charge is not really distributed atom by atom, but different parts of space have different charge.


 7

Formal charge, like oxidation state, is fundamentally just a bookkeeping device (with a different counting method). This being so, formal charge can be correlated with an unequal sharing electrons between like atoms. In such cases it points to molecular polarity in situations where we would not ordinarily expect it.


The classic example is ozone, $\ce{O3}$ (and it's congener, $\ce{S3}$ which is known in sulfur-rich gases such as Io's volcanic eruptions). If we assume that the bonding is nonpolar, then because the central atom shares three of its electrons and the others share only one or two, we see a separation of formal charges and therefore the bonding is not really nonpolar. The middle oxygen has a formal charge of $+1$, the others $-1/2$ apiece. The electrons in the bonds are likely polarized towards the middle oxygen to compensate for this inequity, but the bent ozone molecule still retains some dipole moment ([0.53 D](https://en.wikipedia.org/wiki/Ozone)) correlated with the formal charge separation.


 5