General guideline for drawing structures

Leslie Cheng 4B
Posts: 35
Joined: Fri Sep 28, 2018 12:29 am

General guideline for drawing structures

Hi! Does anyone have a general guideline for drawing lewis or resonance structures? For example, I'm not sure if I should calculate the formal charge for all possible structures first before deciding on which structure is the correct one, because it seems a bit time-consuming during timed tests. Thanks in advance!

Artin Allahverdian 2H
Posts: 76
Joined: Fri Sep 28, 2018 12:26 am

Re: General guideline for drawing structures

For the easy structures u will not have to do much work since there are no exceptions.

However for resonance, u can tell because the molecule will be the same just with the double bond occurring in either side. Calculate the formal charge just to make sure it is resonance.

For the free radical elements like in group 13 of the periodic table, they can either fill up the octet or not fill up the octet. It all depends on how many electrons you have available to work with and also on the formal charge.

For the elements that are in the n=3 energy level or greater, there is a possibility of having more than an octet since the d orbital can increase the number of valence electrons the element can hold. Again, I’d calculate the formal charge to see whether the element should or should not have more than an octet.

In general, calculating the formal charge is the most secure way of making sure you got the right Lewis structure. It may be time consuming, but if u practice a lot calculating the formal charge, you will be able to work much faster on the test.

Julia Lee
Posts: 80
Joined: Fri Sep 28, 2018 12:29 am

Re: General guideline for drawing structures

I think that Lavelle said in lecture that generally, the lower electronegative element will be in the middle of the structure and then the other elements will usually form bonds around it. Also, Hydrogen only can bond once so it is never in the middle of a molecule.

Kelly Zhang 1L
Posts: 31
Joined: Fri Sep 28, 2018 12:18 am

Re: General guideline for drawing structures

when we draw resonance structures, if it has a net overall charge of -2, do we have to bracket the drawing and write subscript -2?

Julia Lee
Posts: 80
Joined: Fri Sep 28, 2018 12:29 am

Re: General guideline for drawing structures

kellyzhang1210 wrote:when we draw resonance structures, if it has a net overall charge of -2, do we have to bracket the drawing and write subscript -2?

Yes, that is correct