Determining which element to give a formal charge

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Keirsten Andersen 3L
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Joined: Fri Sep 25, 2015 3:00 am

Determining which element to give a formal charge

Postby Keirsten Andersen 3L » Thu Dec 03, 2015 12:06 am

When drawing a lewis structure, is there a method to decide which element will receive the formal charge? For example, if I can arrange electrons/ bonds to give either Cl or O a formal charge of 1-, how do I decide which way to draw it? For example when drawing SO2Cl-, you can either attach one O to the central S with a double bond, the second O with a single bond (and formal charge of 1-), the Cl with a single bond and a lone pair on the S OR you can attach the two O's and the Cl to the central S all with double bonds resulting in the Cl having a formal change of 1-. Which one is more stable? Why?

Nitin Joseph
Posts: 21
Joined: Fri Sep 25, 2015 3:00 am

Re: Determining which element to give a formal charge

Postby Nitin Joseph » Thu Dec 03, 2015 3:08 pm

Chlorine can never form a double covalent bond with any element, as it would then have 10 electrons in its orbits, and it cannot hold an expanded octet. Chlorine always forms a single bond.
Now for the oxygens, one would be connected to the Sulfur with a double bond (FC = 0), and the other with a single bond (FC = -1), to the give the ion a charge of -1.

Keirsten Andersen 3L
Posts: 33
Joined: Fri Sep 25, 2015 3:00 am

Re: Determining which element to give a formal charge

Postby Keirsten Andersen 3L » Sat Dec 05, 2015 4:22 pm

Why can't Cl have an expanded octet? Our book states "nonmetal atoms in Period 3 or later periods can expand their valence shells," and Cl is a non metal in period 3.

Jeannie Huang 3B
Posts: 22
Joined: Fri Sep 25, 2015 3:00 am

Re: Determining which element to give a formal charge

Postby Jeannie Huang 3B » Sat Dec 05, 2015 8:35 pm

Although chlorine is in period 3, it's so electronegative that it doesn't like sharing electrons, and usually won't form anything more than one single bond. Thus, it's an exception to the expanded octet rule for periods >= 3.


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