Formal Charge Tricks

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madisondesilva1c
Posts: 91
Joined: Fri Sep 28, 2018 12:16 am

Formal Charge Tricks

Postby madisondesilva1c » Fri Nov 02, 2018 11:29 am

Even though there is a formal charge equation, I find myself coming up with the incorrect formal charge often after drawing the lewis structure and my error is aways in the 1/2(shared) portion of the calculation. Is there any helpful tips or tricks to correctly calculate formal charge or any common mistakes that I can avoid?

Megan Gianna Uy 3L
Posts: 30
Joined: Fri Sep 28, 2018 12:26 am

Re: Formal Charge Tricks

Postby Megan Gianna Uy 3L » Fri Nov 02, 2018 11:33 am

To find the formal charge use the formula FC = V (L+ S/2).
Where FC=formal charge
V= number of valence electrons
L= number of lone electrons that arent bonded
S= number of shared electrons that are being bonded

LaurenJuul_1B
Posts: 65
Joined: Fri Sep 28, 2018 12:17 am

Re: Formal Charge Tricks

Postby LaurenJuul_1B » Fri Nov 02, 2018 12:01 pm

the 1/2 shared portion of formal charge has to do with the shared bonds between electrons. Each bond represents 2 electrons, but since they are being shared, we only calculate with half of this number. (I.e. if two atoms share 2 bonds you take 4(electrons)/2(bonds) to get two. An easier way to think about it may be formal charge = [number of valence electrons on atom] - [non-bonded electrons + number of bonds]. this is because the number of shared electrons divided by 2 is the same thing as the number of bonds attached to the atom. so rather than trying to divide in your calculation (which may be where you are messing up) just use the number of bonds instead.

Aria Soeprono 2F
Posts: 64
Joined: Fri Sep 28, 2018 12:27 am

Re: Formal Charge Tricks

Postby Aria Soeprono 2F » Fri Nov 02, 2018 2:19 pm

In my section, we talked about a shortcut where you take the number of valence electrons the element has in it's normal state, and subtract the number of valence electrons it has when it is bonded.

Erin Kim 2G
Posts: 75
Joined: Fri Sep 28, 2018 12:26 am

Re: Formal Charge Tricks

Postby Erin Kim 2G » Fri Nov 02, 2018 2:30 pm

A shortcut you can use for formal charge is to take the total number of valence electrons and subtract the lines and dots surrounding the atom. so FC= VE -(# of lines/bonds + # of dots).

Kelly Hollman
Posts: 33
Joined: Fri Sep 28, 2018 12:27 am

Re: Formal Charge Tricks

Postby Kelly Hollman » Fri Nov 02, 2018 2:33 pm

You can think about it as formal charge = "should" - "has".
What your atom should have are the number of valence electrons according to the periodic table.
What your atom, when bonded, actually has are: (lone pair electrons) + (1/2)*(the number of bounded electrons).


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