Chemical Bonding: H₂SO₃ Lewis Structure

Drawing the Lewis Structure for H₂SO₃

Viewing Notes:

• In H₂SO₃ Lewis structure Sulfur is least electron electronegative atom and goes in the center of the Lewis structure.
• When we have an H (or H2) in front of a polyatomic molecule (like CO₃, SO₄, NO₂, etc.) we know that it's an acid. This means that the Hydrogen atoms will be attached to the outside of the oxygen molecules. Knowing this information makes it much easier to draw the Lewis structure for H₂SO₃.
• For the Lewis structure for H₂SO₃ you should take formal charges into account to find the best Lewis structure for the molecule.
• In the Lewis structure for H₂SO₃ there are a total of 26 valence electrons.

Transcript: This is the H2SO3 Lewis structure: sulfurous acid. The key to understanding this Lewis structure is recognizing these two H's in front attached to a polyatomic ion. That makes it an acid. And these Oxygens here, the Hydrogens will attach to the outside of the Oxygens. So we'll put our Sulfur here in the middle, it's the least electronegative. We have three Oxygens. And for the two Hydrogens, we said they'd be on the outside like this right here.

We have a total of 26 valence electrons for the H2SO4 Lewis structure. We'll put 2 electrons between atoms to form the chemical bonds there. We've used 10 valence electrons. Then we'll fill the octets on the Oxygens. So you have 10, 12, 24, and then back to the center, 26. So we've used all 26 valence electrons, and everything in the structure, in the H2SO4 Lewis structure, has a full outer shell. So it looks like a pretty good Lewis structure.

The thing is, when you see Sulfur, Sulfur is in period 3 on the periodic table, Sulfur can hold more than 8 valence electrons. So we really need to look at the formal charges for this molecule. When we calculate the formal charges, the Sulfur has a +1 charge, and the Oxygen has a -1 charge. So we'd like our formal charges to be as close to 0 as possible. And to do that, we can move this pair of valence electrons between the Oxygen and the Sulfur to form a double bond. By doing so, we're still only using 26 valence electrons, but now the formal charges on all of the atoms are 0. That makes for a much more stable Lewis structure for H2SO4. So if you see the Sulfur there, you'll want to check your formal charges.

This is Dr. B., thanks for watching.