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