Tips on drawing out Lewis Structures?

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Joana Ramirez 1F
Posts: 4
Joined: Fri Sep 25, 2015 3:00 am

Tips on drawing out Lewis Structures?

Postby Joana Ramirez 1F » Fri Oct 16, 2015 11:52 pm

I did not learn how to draw complicated structures for compounds in my high school chem class (i.e. adding double/triple bonds, contributors, taking in account ions, etc.), and so it is somewhat difficult for me to do now. Any advice in drawing out Lewis structures quickly and easily?

kshalbi
Posts: 68
Joined: Fri Sep 25, 2015 3:00 am

Re: Tips on drawing out Lewis Structures?

Postby kshalbi » Sat Oct 17, 2015 1:46 am

Hi Joana! When drawing out Lewis Structures, you want to follow these steps:

1. The most electronegative atom goes in the middle (there are a few exceptions). Electronegativity increases when going across a period and up a group.
2. Figure out how many valence electrons each atom has and add these values up. Don't forget to take into account if the molecule has an overall charge. For example, if the molecule has a -2
charge, then you add 2 electrons to the total number of electrons that you already have. If the molecule has a +2 charge, then you subtract 2 electrons from the total number of electrons that
is supposed to be represented by your diagram.
3. Draw a single bond from each of the surrounding atoms to the central atom and add lone pairs to fulfill the octet rule. Remember though that there are exceptions to this rule. Some atoms like
Sulfur and Phosphorous can have more than and octet, while Boron can have less than an octet (it had 6 valence electrons).
4. Count the number of electrons represented in your diagram. Remember that a bond represents two electrons. After counting all the electrons, make sure it matches up to the first value you
calculated when you added up all the valence electrons from each atom. If the number of electrons represented in your diagram is greater than the number that was originally calculated, draw
double bonds in place of some of the single bonds. Again, make sure the octet rule is satisfied where it applies.
5. Often times there are multiple structures possible for a molecule. To figure out the best one, you must calculate the formal charge of each atom. To do this, you use the formula FC=V-(L+B/2),
where "FC" means Formal Charge, "V" represents the number of valence electrons in the atom, "L" represents lone electron pairs around the atom of interest, and "B" stands for the number of
bonded electrons.
6. After calculating the formal charge of each atom in each arrangement, the best structure will be that which has the lowest formal charge values for each atom.


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