Determining lowest energy lewis structure

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Lauren Bui 1E
Posts: 54
Joined: Sat Jul 20, 2019 12:15 am

Determining lowest energy lewis structure

Postby Lauren Bui 1E » Fri Nov 01, 2019 8:33 pm

Is there a strategy for drawing the lowest energy lewis structure or do you have to guess and check? Are there any tips for rearranging the structure to have the lowest formal charge?

Cassidy Kohlenberger 3D
Posts: 51
Joined: Wed Sep 18, 2019 12:19 am

Re: Determining lowest energy lewis structure

Postby Cassidy Kohlenberger 3D » Fri Nov 01, 2019 8:42 pm

Personally, I like to draw out the Lewis structure with single bonds first, fulfilling the octet rule for the atoms if possible. Then I calculate formal charge: a quick way to do this is by drawing a circle around the atom (basically splitting the bonds in half) and counting how many electrons it has. Then subtract from the number of valence electrons that atom typically has. Then you can rearrange the electrons into double bonds or whatever you need to make each atom's formal charge as close to zero as possible.

Ryan Narisma 4G
Posts: 104
Joined: Fri Aug 30, 2019 12:18 am

Re: Determining lowest energy lewis structure

Postby Ryan Narisma 4G » Fri Nov 01, 2019 8:44 pm

Hi Lauren Bui 1E! To answer your question, when drawing the lowest energy lewis structure try to arrange the bonds in such a way that all (if not then the majority) of the atoms to have a formal charge of 0. When all (or most) formal charges are 0, the lewis structure is said to be the most stable—in other words, the lowest energy. There are a few patterns you may notice when drawing a lot of structures:
-Oxygen really likes it when it has a double bond with another atom (because the formal charge of Oxygen will be 0)
-Carbon's formal charge is 0 when it can make 4 bonds (it is tetravalent)
-Nitrogen likes it when it has 3 bonds and a lone pair (because the formal charge will be 0)
-All the halogens (F, Cl, Br, I) prefer to have a single bond with another atom because it's formal charge will be 0
-Period 3+ elements can have an expanded octet and can make more bonds between atoms which is helpful in reducing the amount of formal charges within the molecule
-Remember to always have the central atom be the least electronegative (Hydrogen and Halogens will most likely not be the central atom)

I hope these tips help!

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