H3PO3 (Phosphorous acid) Lewis Structure | VSEPR Theory

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 | H3PO3

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.

Lewis structure of H3PO3

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.

H3PO4 lewis structure

VSEPR theory - Steps of drawing lewis structure of H3PO3

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.

  1. Find total number of electrons of the valance shells of phosphorous, oxygen and hydrogen atoms
  2. Total electrons pairs
  3. Center atom selection
  4. Put lone pairs on atoms
  5. Check the stability and minimize charges on atoms by converting lone pairs to bonds until most stable structure is obtained.

Drawing the correct lewis structure is important to draw resonance structures correctly

Total number of electrons of the valance shells of H3PO3

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.

  • Total valence electrons given by Phosphorous atom = 5

There are three oxygen atoms in H3PO3 molecule, Therefore

  • Total valence electrons given by oxygen atoms = 6 *3 = 18

There are three hydrogen atoms in H3PO3 molecule, Therefore

  • Total valence electrons given by hydrogen atoms = 1*3 = 3

There are no charges in H3PO3 molecule.

  • Total valence electrons = 5 + 18 + 3 = 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, H3PO3 molecule, Total pairs of electrons are 13.

Center atom and sketch of H3PO3 molecule

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.

H3PO3 sketch

Mark lone pairs on atoms

  • There are already three P-O bonds, two O-H bonds and one P-H bond in the above sketch. Therefore only seven (13-6 = 10) valence electrons pairs are remaining to draw the rest of the structure.
  • Start to mark remaining seven valence electrons pairs as lone pairs on outside atoms (on oxygen atoms). All remaining valence electrons pairs can be marked on oxygen atoms. (oxygen atom cannot keep more than eight electrons in its valence shell).
  • Therefore, there is no electron pairs to mark on phosphorous atom.
mark electron pairs on atoms H3PO3

Charges on atoms

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.

mark charges on atoms in H3PO3

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

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.

  • Oxygen atoms should hold negative charges because electronegativity of oxygen is higher than phosphorous. Otherwise, we can say, ability of holding negative charges is great in oxygen atoms than phosphorous atoms.
  • The drawn structure has -1 charges on one oxygen atom and +1 charge on phosphorous atom.
  • Now, we should try to minimize charges by converting lone pair or pairs to bonds. So convert one lone pair of the oxygen atom to make a new P-O bond.
  • Now there is a double bond between phosphorous atom and one oxygen atom. Now, there are two P-O single bonds between phosphorous atom and other two oxygen atoms.
convert lone pairs to bonds in H3PO3

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.


Lewis Structures of Molecules