# Drawing Lewis Structures

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#### DarkVoid11

##### Full Member
2+ Year Member
7+ Year Member
Hi guys,

I'm having trouble mentally drawing the lewis structures of molecular compounds that break the octet rule.
I understand the steps of counting the number of valence electrons, creating the single bonds, filling the outer atoms, and the left over electrons are placed in the center atom.

But with molecules like SO3 or SO3^-2 they confuse me.
Why do all three oxygens double with the sulfur in SO3?

The way that I was taught is that you should always try to limit formal charge on the molecule. This would imply that you want to use double bonds to get rid of the negative charge on the oxygen. Another thing that you can look at is the idea of resonance. With three "double bonds" you can share the double bonds across the entire molecule. With 2 and a lone pair this isn't as favorable.

Someone correct me if I am wrong with this.

The way that I was taught is that you should always try to limit formal charge on the molecule. This would imply that you want to use double bonds to get rid of the negative charge on the oxygen. Another thing that you can look at is the idea of resonance. With three "double bonds" you can share the double bonds across the entire molecule. With 2 and a lone pair this isn't as favorable.

Someone correct me if I am wrong with this.

Thanks! Would it be correct to say that we would always try to make the formal charge of the central atom equal to zero?
But if anything, does determining formal charges seem too time consuming especially if I have a question that compares 5 different molecules and determining their polarity? If that makes sense.

Are you trying to determine if the molecule is polar?

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Are you trying to determine if the molecule is polar?

Yes out of 5 molecules. How would you go about questions like that? They just ask "which of the following molecules are polar". realistically, I just feel like this would be too time consuming but it may be my approach

Ah. I personally memorized a table. There are only certain shapes that can be nonpolar and that is only if all of the molecules surrounding the central atom are the same. Double bonds make things trickier but if you can narrow it down to only having to look at a few that makes it a little easier.

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Ah. I personally memorized a table. There are only certain shapes that can be nonpolar and that is only if all of the molecules surrounding the central atom are the same. Double bonds make things trickier but if you can narrow it down to only having to look at a few that makes it a little easier.

You're talking about the molecular geometry right? What if you came across a molecule that you didn't recognize. Would you spend 30 secs or more to mentally/draw out the lewis structure?

I guess personally at that point I would probable give a best guess, mark and move on. I would then come back if I have time. Personally though I am much rustier on Ochem and will need all the time I can get for that section.

I guess personally at that point I would probable give a best guess, mark and move on. I would then come back if I have time. Personally though I am much rustier on Ochem and will need all the time I can get for that section.

Correct me if I'm wrong, but when you go through the science section, we have 90 minutes for the three natural science sections. May I come back to something I marked in Gen chem or Biology when I am finish with O chem?

I believe that is correct. It is the natural sciences section so there is no stop between subjects, meaning anything you want to go back to in any of the science sections you can within that 90 minute time limit.

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I believe that is correct. It is the natural sciences section so there is no stop between subjects, meaning anything you want to go back to in any of the science sections you can within that 90 minute time limit.

As always, thanks for your input

1 user
Hi guys,

I'm having trouble mentally drawing the lewis structures of molecular compounds that break the octet rule.
I understand the steps of counting the number of valence electrons, creating the single bonds, filling the outer atoms, and the left over electrons are placed in the center atom.

But with molecules like SO3 or SO3^-2 they confuse me.
Why do all three oxygens double with the sulfur in SO3?
I am going to give you a nice rule with Sulfur that I tell all my students. Sulfur LOVES.....Loves, do you hear, to have double bonds !!! This atom can utilize d orbitals to expand the normal 8 electrons we see in the octet rule. After you count the total valence electrons, try to first put in some double bonds. Molecules such as S02, S03, and even S04 -2 ion all contain double bonds. Other atoms that like to have a double bond are carbon and nitrogen. Occasionally, P too. I hope this helps. If not, The Zumdahl,Ebbing, Brown and LeMay or Raymond Chang text has other fine examples.

Dr. Jim Romano

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I am going to give you a nice rule with Sulfur that I tell all my students. Sulfur LOVES.....Loves, do you hear, to have double bonds !!! This atom can utilize d orbitals to expand the normal 8 electrons we see in the octet rule. After you count the total valence electrons, try to first put in some double bonds. Molecules such as S02, S03, and even S04 -2 ion all contain double bonds. Other atoms that like to have a double bond are carbon and nitrogen. Occasionally, P too. I hope this helps. If not, The Zumdahl,Ebbing, Brown and LeMay or Raymond Chang text has other fine examples.

Dr. Jim Romano

Thanks Dr Romano!

1 user