Question

What is the best Lewis structure below?

Answer 1

The best lewis structure is B

Explanation:

Let's have a look at the rules how we set up lewis structures

(most) atoms want to have 8 electrons (or 2) in the outer shell (the valence shell). This is called the octet rule

1. Count all the valence electrons from the atoms involved in your structure. Divide this number by 2 to obtain the number of electron pairs.
2. Calculate how many electrons every atom needs to fill their outer shell up to 2 electrons for `H` and `He`, and 8 electrons for all the other atoms. Divide this number by 2 to obtain the number of electron pairs.
3. Calculate the number of bonding electrons pairs by=
   Needed `e^-`pairs `-` Valence `e^-`pairs =number bonding `e^-` pairs
   (or: step 2 `-` step1 = step3)
4. Calculate the number of nonbonding electron pairs by
   Valence `e^-` pairs `-` bonding `e^-` pairs= number non-bonding `e^-`pairs.
   (or: step1 `-` step3=step4)
5. Then we have to calculate the formal charge of every atom in the structure.
   Formal charge = valance `e^"-" -0.5 xx` bonding `e^"-"-` nonbonding `e^-`

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Now let's have a look at your structures

A) One of the C atoms does not obey the octet rule and therefore the structure is not correct.
B) This structure seems correct
C) The O-atom has 4 bonding electron pairs around it. The formal charge of this atom is, therefore, `+2`, which is not favourable for the electronegative O-atom. The carbon atom in the middle has the charge `-2`, which makes this structure not very likely. Structures with less separation of opposite charges are more likely than those with more charge separation.
D) The O-atom does not obey the octet rule since it has 10 electrons in the outer shell.

Practice makes these kinds of problems more easily, because you will instantly see that an O-atom is not very likely to have 4 bonds, and carbon will most likely have 4 bonds.