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### How to know where each atom should go for bonding?

Posted: Mon Oct 28, 2019 12:41 am
I'm confused about writing Lewis structures for polyatomic molecules. I understand how to calculate the number of valence electrons and bonds, but how do you know exactly where each atom should go? Like for acetic acid, how do you know what shape it's suppose to be?

### Re: How to know where each atom should go for bonding?

Posted: Mon Oct 28, 2019 12:48 am
I am not exactly sure on the general rules for this. But for this specific example, carbon likes to have four bonds so they are in the middle and are bonded to the most elements. Hydrogen likes to only form one bond so that the first shell is completely filled, which explains why each hydrogen is only bonded to one other atom in this case.

### Re: How to know where each atom should go for bonding?

Posted: Mon Oct 28, 2019 1:24 am
In Acetic Acid, $CH_{3}COOH$ or $CH_{3}CO_{2}H$, you can see two groups: CH3 and COOH.

Elements like C and O usually take the "inside" of the structure while atoms of hydrogen will take the exterior as they bond with C and O.
In CH3, you can assume that C takes the center and that it's surrounded by 3 H atoms. Since Carbon has 4 ve- (valence electrons) and it will need 4 more e- to achieve a "full" state of 8 ve- you can come to the conclusion that 1 more electron is needed for stability since we have been given 3 electrons from the 3 H atoms. You can now bond the C of CH3 to the C of COOH. One ve- of the C in COOH can be "shared' with the C of CH3 so it will have a total of 8ve-.

COOH is something called a "functional group", and it known in organic chemistry for taking a specific structure. It's easiest to memorize the shape of this functional group in order to apply it to the rest of Acetic Acid.

I recommend this video: https://www.youtube.com/watch?v=5vOl_dVkmQQ

### Re: How to know where each atom should go for bonding?

Posted: Tue Oct 29, 2019 8:41 am
If you are looking at examples such as ONF from question 2B.1, you would see that to get a zero formal charge, O needs two bonds, N needs three, and F needs 1. Since N has the most bonding possibilities, N would be the central atom, and the O and F would be around it.