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It depends on the context! Priority wise you would want to use double bonds to match the number of electrons with the total amount you are supposed to have. Then comes the necessity to have formal charges of zero for each atom. Hope this helps!
When figuring out whether to place a double or triple bond you should always look at the number of valence electrons present as well as the number of bonds a central atom is likely to form. Another good way to know whether to use double or single bonds is to calculate the formal charge on each atom in the molecule. The less charges on each atom that there are, the more stable the molecule is. If a double or triple bond can be placed to lessen the number of different formal charges, then it should be added.
Thus far, I have been calculating formal charge on the central atom each time with a single versus a double bond in order to determine which would be the more stable structure. I feel like there's an easier way to do this. How does looking at valence electrons factor into it? In other words, how do I know that-- say, in COCl2-- carbon forms a double bond with oxygen and there aren't just lone electrons in the central atom (which is carbon)? Is the easiest way to do this always calculating formal charge with one bond and the formal charge with two bonds and comparing the two? (Sorry for the long question-- just trying to articulate clearly :))
When I finish drawing my Lewis structure, I count the electrons to make sure it matches the number of electrons i should be using. If the number of electrons I used is higher and all the atoms have octets around them this generally means you should add a double or triple bond somewhere. Of course this strategy only works if all the elements in the molecule follow the octet rule. If the central atom is an element that violates the octet rule, I plan my Lewis structure around the formal charge of the central atom.
You should start drawing your Lewis structures with a single bond and then fill the octets. If they are full and you still have electrons that need to be added then add a double bond and try to use the electrons that you need to. If you can’t fill the octets due to a lack of electrons then try using a double bond and go from there. Also, make sure to check that the formal charges at the end are correct
What I usually do is create the lewis structure by starting with all single bonds and drawing in the valence electrons, then if I notice the electrons are unpaired, then I'll start creating bonds, then I'll focus on the formal charges. I always try to see that the central atom has a formal charge closest to 0, so I'll shift the bonds even more to create the formal charges into 0.
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