Phosphorous acid (H3PO3) lewis structure can be built by following VSEPR theory. In the VSEPR theory, electrons in the valence shells of atoms, charges on atoms, number of lone pairs exist in an atom are considered. Charges of atoms should be minimized in the lewis structure of H3PO3 to be a stable structure.
Phosphorous acid is a dibasic weak acid. It means, it can release two hydrogen atoms to show acidic characteristics. Therefore, we can assume, there should be two -OH bonds in phosphorous acid molecule.
Most stable and probable lewis structure of H3PO3 is shown below. We can start remaining construction of lewis structure after we identified the best sketch.
Following steps are followed to draw the H3PO3 lewis structure and they are explained in detail in next sections. If you are are beginner to lewis structure construction, follow these sections slowly and properly to understand it completely.
Drawing the correct lewis structure is important to draw resonance structures correctly
Phosphorous is a group VA elemet. Therefore, it has five electrons in its valence shell. Oxygen has six electrons in its last shell because it is a group VIA element. Hydrogen has only one electron in its valence shell. To find number of valence electron , these valence electrons of each element should be multiplied with respective number of atoms. Below, That step is done.
There are three oxygen atoms in H3PO3 molecule, Therefore
There are three hydrogen atoms in H3PO3 molecule, Therefore
There are no charges in H3PO3 molecule.
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, H3PO3 molecule, Total pairs of electrons are 13.
There are some requirements to be the center atom. Having a high valence is a main requirement to be a center atom. For H3PO3 molecule, phosphorous has the highest valence than oxygen and hydrogen.
Because, phosphorous acid is a dibasic acid, it can release two H+ ions in the water. Therefore, there should be two -OH groups in H3PO3 molecule. Therefore, we can present following sketch for H3PO3 molecule.
Charges on atoms is important to find the most stable lewis structure. Therefore, we should try to find charges if there are.
After, marking electron pairs on atoms, we should mark charges of each atom. One oxygen atoms will get a -1 charge and phosphorous atom get a +1 charge. Phosphorous acid is a neutral molecule and overall charge should be zero. The overall charge of th e molecule is ( -1*1 + (+1) ) = 0.
When charges exist on lot of atoms in an ion or molecule, that structure is not stable. We should try to reduce charges on atoms if there is a possibility. Now, we are going to reduce charges on drawn structure.
You see charges of atoms are reduced. Now, there are no charges in atoms. therefore, this structure should be a the lewis structure of phosphorous acid.