Formal Charge

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Harjot Manku 1C
Posts: 21
Joined: Fri Sep 20, 2013 3:00 am

Formal Charge

Postby Harjot Manku 1C » Sun Jul 09, 2017 10:18 pm

Is the formal charge just supposed to help us make a more stable Lewis Structure?

Kyle Sheu 1C
Posts: 87
Joined: Fri Jun 23, 2017 11:39 am

Re: Formal Charge

Postby Kyle Sheu 1C » Sun Jul 09, 2017 11:29 pm

Correct, it is a method we use to help identify the most stable structure.

604744616
Posts: 34
Joined: Sun Jun 25, 2017 3:00 am

Re: Formal Charge

Postby 604744616 » Mon Jul 10, 2017 8:09 am

With the formal charge how do we know exactly where it stands in the Lewis structure? And do we just need to see if the compound has a formal charge then it is most likely to be put in the Lewis structure?

Ben Rolnik 1D
Posts: 33
Joined: Fri Jun 23, 2017 11:39 am

Re: Formal Charge

Postby Ben Rolnik 1D » Sat Jul 15, 2017 2:53 pm

so here's the formula.

1. count the valence electrons for each atom in the molecule

2. place the least electronegative atom in the center (or the atom with the lowest ionization energy... or the atom farthest away from Fluorine).

3. arrange all the other atoms around it symmetrically (this will also help in terms of determining the molecules shape later).

4. place a single bond connecting all the outer atoms to the central atom.

5. Add free electrons to all the surrounding atoms to form octets until you've reached the # in [1]. At this stage if everything has a perfect octet, great! Otherwise look at the rules for which atoms can form LESS than an octet (e.g. B) and which can form MORE than an octet (e.g. S).

6. Calculate the formal charge. This equation is: FC = (Valence Electrons -- the # you calculated in 1) - (free electrons + # of bonds [or s/2]). This equation gives you the formal charge.... the formal charge of atoms like Oxygen with a single bond is always "-1".... so if you can add a double bond to the oxygen you can reduce the FC for that molecule.

7. Rearrange the bonds (or add more bonds) so that you create the lowest formal charge possible (this create the most stable atom.)

8. If you find there are multiple ways to draw the lowest formal charge then you have resonance hybrids -- and the REAL structure of the atom is a synthesis of all those structures.

Make sense?

Natalie Espadas 1A
Posts: 24
Joined: Wed Nov 16, 2016 3:03 am

Re: Formal Charge

Postby Natalie Espadas 1A » Sat Jul 15, 2017 11:54 pm

to clarify, the lowest numbers mean the structure is most stable right? I am unsure of this but I would appreciate if somebody can clarify this for me. Thank you in advance!

Kyle Sheu 1C
Posts: 87
Joined: Fri Jun 23, 2017 11:39 am

Re: Formal Charge

Postby Kyle Sheu 1C » Sun Jul 16, 2017 12:57 pm

Correct, the set of formal charges closest to zero (think of it as smallest absolute value) give the most stable Lewis structure.

However, all possible structures contribute to the actual resonance structure, and some (e.g. the more stable structures) more so than others.


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