Sulfurous Acid (H2SO3) Lewis Structure

Lewis structure of sulfurous acid (H2SO3) contains two O-H bonds,one S=O bond, two S-O bonds.. Also, there is one lone pairs on sulfur atom. Concept of number of total valence electrons of atoms are used to draw lewis structure of H2SO3. Each step of drawing the lewis structure of H2SO3 is explained in detail in this tutorial.

H2SO3 lewis structure

H2SO3 lewis structure

Sulfur atom is the center atom in H2SO3 molecule. Three oxygen atoms are located around the sulfur atom. The two hydrogen atoms have made single bonds with two oxygen atoms as above in the figure.

Steps of drawing lewis structure of H2SO3

There are several steps to draw the lewis structure of H2SO3. Those steps are explained in detail in this tutorial. Because H2SO3 molecule is a bit complex molecule, almost all steps may be used. Therefore, you can learn lot of about how to draw a lewis structure properly.

steps of drawing H2SO3 lewis structure

  1. Find total number of electrons of the valance shells of hydrogen atoms, oxygen atoms and sulfur atom
  2. Total electrons pairs as lone pairs and bonds
  3. Center atom selection
  4. Mark lone pairs on atoms
  5. Mark charges on atoms if there are charges.
  6. Check the stability and minimize charges on atoms by converting lone pairs to bonds to obtain best lewis structure.

Total number of electrons of the valance shells of H2SO3

There are three elements elements in H2SO3; hydrogen, oxygen and sulfur. Hydrogen is a group IA element and has only one electron in its last shell (valence shell). Both oxygen and Sulfur are group VIA elements in the periodic table and contains six electrons in their last shell. Now we know how many electrons are includes in valence shells of hydrogen and sulfur atom.

  • valence electrons given by hydrogen atoms = 1 * 2 = 2
  • valence electrons given by oxygen atoms = 6*3 = 18
  • valence electrons given by sulfur atom = 6*1 = 6

  • Total valence electrons = 2 + 18 + 6 = 26

Total valence electrons pairs

Total valance electrons pairs = σ bonds + π bonds + lone pairs at valence shells

Total electron pairs are determined by dividing the number total valence electrons by two. For, H2SO3, Total pairs of electrons are 13 in their valence shells.

Selection of center atom and sketch of H2SO3 molecule

To be the center atom, ability of having greater valance is important. Then, from hydrogen, oxygen and sulfur, which atom has the highest valence? Maximum valence of sulfur is six. Hydrogen's only valence is one and oxygen's maximum valence is two. Therefore, sulfur atom should be the center atom of H2SO3. Now, we can construct the sketch of H2SO3 to show how atoms are located in the molecule.

H2SO3 sketch

Lone pairs on atoms

After deciding the center atom and sketch of H2SO3 molecule, we can start to mark lone pairs on atoms. Remember that, there are total of thirteen electron pairs to mark on atoms.

  • There are already five sigma bonds in the above drawn sketch. Now only eight (13-5) electron pairs are remaining to mark on atoms.
  • Usually as a theory, those remaining electron pairs should be first marked on outside atoms. In H2SO3, hydrogen atoms and oxygen atoms are the outside atoms. Hydrogen atoms cannot keep more than two electron in its last shell. There are already two electrons in hydrogen atoms in our drawn sketch. Therefore, we cannot mark those two electrons pairs on hydrogen atoms.
  • Therefore, then mark those two electrons pairs on oxygen atoms. When marking lone pairs, remember to protect octal rule. All three oxygen atoms will take seven lone pairs. Now, only one lone is remaining and it is marked on sulfur atom as the following figure.
mark lone pairs on atoms in H2SO3

Mark charges on atoms

According to the above sketch, sulfur atom has a +1 charge and one oxygen atom has -1 charge. Other two oxygen atoms do not have charges.

mark charges on atoms in H2SO3

Check the stability and minimize charges on atoms by converting lone pairs to bonds

If there are charges on atoms and if those charges can be reduced by converting lone pairs to bonds, we should do that to obtain the best stable lewis structure. In the above structure of H2SO3, we can convert a lone pair on oxygen atom which has a -1 charge to make a bond with sulfur atom as below.

reduce charges on atoms in H2SO3

In the above structure, there are no charges on atoms. Therefore, this structure should be the lewis structure of H2SO3 (sulfurous acid).


a skeletal structure for sulfurous acid shown below. starting from this structure, complete the lewis structure that follows the octet rule on all atoms.

Skelton of sulfurous acid is given above. In sulfurous acid, there are 10 electrons around sulfur atom where octet rule is not followed. It is not necessary for atoms such as sulfur because they can keep more than eight electrons in their vaalence shell.

By observing lewis structures of H2SO3, can we say whether it is a weak acid?

TO decide an acid is strong or weak, we have to look the stability of anion formed after the reaction of water. H2SO3 forms SO32- ion which is an unstable ion in water. Therfore, we consider H2SO3 is a weak acid.

How many lone pairs and double bonds on sulfur atom according to the lewis structure of H2SO3?

There is only one lone pair on sulfur atom in H2SO3 lewis structure. There is a one double bond between sulfur atom and oxygen atom.

Lewis Structures of Molecules