Drawing the Lewis Structure for SO32-

Viewing Notes:

  • The Lewis structure for SO32- is requires you to place more than 8 valence electrons on Sulfur (S).
  • You might think you've got the correct Lewis structure for SO3 at first. Remember, Sulfur is in Period 3 and can hold more than 8 valence electrons.
  • You'll want to calculate the formal charges on each atom to make sure you have the best Lewis structure for SO3. You'll need to place a double bond on the Oxygen atom in order to have the best Lewis structure.
  • When you're done put brackets, along with 2-, around the Lewis structure for SO32- to show that it is an ion with a negative two charge.
  • SO32- has a total of 26 valence electrons (remember that -2 charge counts as two valence electrons).

Transcript: Hi, this is Dr. B. Let's do the SO3 2- Lewis structure. For the SO3 2- compound, we have 26 total valence electrons, and that includes these two electrons up here--there are two extra valence electrons. So we have 26. Let's put the Sulfur at the center and the Oxygens around the outside. Put two electrons between the atoms to form chemical bonds. We've used 6. And then around the outside atoms. So we have 6, 8, 10, and 24. We have two left over. We'll put them right here on the central atom.

So we've used all 26 valence electrons. The structure looks pretty good. Each of the atoms has an octet. We've used the 26 valence electrons. So this is a possible structure for the sulfite ion, SO3 2-. Since Sulfur is in the third period on the periodic table, it can hold more than eight valence electrons. So we should check our formal charges here to see if this is the best structure. For the Sulfur, on the periodic table, it has 6 valence electrons. We have here, on the Lewis structure, 2 nonbonding; and then these right here are all involved in bonds, so 6 over 2. Six minus 2 minus 6 over 2 is a positive 1 for the central Sulfur. For the Oxygens; and they're all the same, so we'll just do one; on the periodic table, 6 valence electrons. Nonbonding, we have 6 nonbonding for each of the Oxygens, and then 2 bonding. Six minus 6 minus 1 gives us -1 for each of the Oxygens. So this makes sense. I have a plus one here, and then three of the minus ones, and that all adds up to negative 2.

So this makes sense as a structure. I can put brackets around it and show that it is, indeed, an ion. I see this plus one here on the central Sulfur, and I know if I pull a pair of electrons from the Oxygen and form a double bond, that'll go to zero. So I'd like to try that and see what it looks like. So here, I've taken these two valence electrons out here and I've moved them in to form a double bond with the Sulfur. That results in this Oxygen here and the Sulfur now having zero for their formal charge. These remain at a negative one, which makes sense; we have a -1, -1, that keeps our -2 right here. We've still used the number of valence electrons that we were initially given. So this may actually be a better Lewis structure for SO3 2- because the formal charges, we have more formal charges at zero than we did in the previous structure.

So that's the SO3 2- Lewis structure. This is Dr. B., and thanks for watching.