formal charge?

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Alondra Juarez section 1E
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Joined: Fri Sep 29, 2017 7:03 am

formal charge?

Postby Alondra Juarez section 1E » Sun May 13, 2018 11:36 am

I understand how to draw a lewis structure but how would I come to do this if there are two elements such as CCL4 and what is meant by the formal charge? this might be simpler than I am making it out to be but I'm quite confused.

Tarek Abushamma
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Joined: Fri Apr 06, 2018 11:01 am

Re: formal charge?

Postby Tarek Abushamma » Sun May 13, 2018 12:43 pm

Formal charge can be calculated using the formula: (# of valence electrons on an atom) - (non bonded electrons + number of bonds).

Anna De Schutter - 1A
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Joined: Wed Feb 21, 2018 3:01 am

Re: formal charge?

Postby Anna De Schutter - 1A » Sun May 13, 2018 2:42 pm


To answer your first question (how to do the Lewis structure for CCl4), the way I do it is I first count the number of valence electrons present in the molecule.
So in CCl4, carbon has 4 valence electrons and chlorine has 7 valence electrons. Because there are four Cl in the molecule CCl4, the total number of valence electrons will be 4electrons (from C) + 4 x 7 electrons (from each Cl element) = 32 electrons.

The best way to then start drawing the Lewis structure is to put the element with the lowest ionization energy in the middle, in this case that would be carbon. This is because the trends of ionization energy in the periodic table are: increase in ionization energy as we go from the left of the periodic table to the right (across a period), and decrease in ionization energy as we go from the top to the bottom of a column (down a group). Because of the concept of diagonal relationship, carbon and phosphorus have the "same" ionization energy and because chlorine lies more to the right of the periodic table than phosphorus, chlorine will have a larger ionization energy than phosphorus and thus carbon (I believe that's how you can justify whether carbon or chlorine has the lowest ionization energy, because of the diagonal relationship, but I'm not too sure).

As we put carbon in the middle of the molecule, we put the 4 chlorine elements around the central carbon and link them to the carbon with single bonds. Because each single bond represents 2 electrons, we have 32electrons - 4 x 2electrons (from the bonds) = 24 electrons left.

We then make sure each element has 8 electrons surrounding them (to conform to the octet rule). Carbon already does so we draw 6 dots around each chlorine element (one dot represents one electron) and we end up with 24 electrons - 4 x 6electrons (from the dots around the Cl element) = 0 electrons which is what we want!

Also, the formal charge refers to how an atom's electrons are distributed in a certain Lewis structure. Another way to calculate it would be:
formal charge = number of valence electrons - number of lone pair electrons - 0.5 x number of bonding electrons
where the lone pair electrons refer to electrons not involved in any form of a bond and bonding electrons refer to electrons involved in a bond. The reason we multiply the number of bonding electrons by 0.5 is because half of the electrons in a bond "belongs" to one of the two elements, and the other half "belongs" to the other element.

I hope this helps! :)
Anna De Schutter - section 1A

Ellie Tsang 1I
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Re: formal charge?

Postby Ellie Tsang 1I » Sun May 13, 2018 2:50 pm

Formal charge also helps when you are drawing Lewis Structures because you want all of the formal charges for each element to be as close to 0 as possible.

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