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Jump to one of the following Lewis structures' parts:   Choose One Introduction to Lewis Structures Part 1- Basics Part 2- Multiple Bonding and Polyatomic Ions Part 3- Resonance Part 4- Hypervalency Part 5- Formal Charge I Part 6- Formal Charge II Part 7- Electron Deficiency Part 8- Oxoacids Part 9- Quizzes

Up until now, the goal has been to draw the Lewis structure. Once you figured it out according to the rules, that was it. Even when working out resonance structures in an earlier section you decided what to draw based on the final structure you wrote. For example, the NO₃^-1 ion resonance structures are:

If you were simply asked to draw the Lewis structure then any of the above would suffice (or any variant on the skeleton structure that's directly analogous).

What happens if you have a choice? What if you have multiple structures that are truly different and not simply a skeleton structure variant or a series of resonance structures in the same manner as those above for the nitrate ion?

How about carbon dioxide- CO₂?

Pull up the Periodic Table if you need one.

Here's a current list of the rules.

CO2
Your turn- enter your answer in the first box and hit "Verify" to see how you did.
1.	Total number of valence electrons.
2. Draw the skeleton structure. 3. Connect the atoms by making bonds. 4. Satisfy the octet rule for the surrounding atoms.

At this point, you've got some decisions to make. Although the surrounding oxygens have a complete octet, the central carbon is 2 pairs of electrons short. Working off of the Lewis structure determined in step 4, above, you need to share 2 pairs of electrons. But, how do you do it? If you grab 2 pairs from one of the oxygen atoms, you get one of the following-

If you share 1 pair of electrons from each oxygen with carbon you get the following-

So, which one is the best Lewis structure? To determine the best structure, we take the previous topic of formal charge and apply it.

Just as a reminder, let's take a quick look at how formal charge is calculated-

We've got 3 Lewis structures to work with for CO₂; but, to simplify things, I'm going to use the first and the third of the following structures since the middle one is a mirror image of the first one:

1.

2.

3.

Let's first tackle the first Lewis structure for CO₂:

Your turn- enter your answer for the formal charges and hit "Verify"
Starting with this Lewis structure:
Formal charge for the left oxygen.
Formal charge for carbon.
Formal charge for the right oxygen.
Sum up the formal charges.

Let's now tackle the third Lewis structure for CO₂:

Your turn- enter your answer for the formal charges and hit "Verify"
Starting with this Lewis structure:
Formal charge for both oxygens.
Formal charge for carbon.
Sum up the formal charges.

As expected, all the structures for carbon dioxide have formal charges which sum up to zero (CO₂ is a neutral molecule). Which one would we use for the best Lewis structure?

Notice that the formal charges for the third structure are all zero unlike those for the other 2 structures. When you choose among various structures, you choose the one that has the smallest number of formal charges. Therefore, you'd pick the third structure as the best Lewis structure:

We need to amend the list of rules for using formal charge to determine the best Lewis structure.