H3PO2 (Hypophosphorous acid) Lewis Structure | VSEPR Theory

Lewis structure of hypophosphorous acid (H3PO2) can be built by following rules of VSEPR theory. When, drawing the lewis structure there are requirements to be satisfied. Charges of atoms should be minimized in the lewis structure of H3PO2 to be a stable structure.

Hypophosphorous acid | H3PO2

Hypophosphorous acid is a monobasic weak acid. It means, it can release only single hydrogen atoms to show acidic characteristics. Therefore, we can assume, there should be one -OH bonds in phosphorous acid molecule.

Lewis structure of H3PO2

Most stable and probable lewis structure of H3PO2 is shown below. In the lewis structure of H3PO2, there is a double bond between phosphorous atom and oxygen atom, two P-H bonds and one O-H bond.

H3PO2 lewis structure

VSEPR theory - Steps of drawing lewis structure of H3PO2

Following steps are followed to draw the H3PO2 lewis structure. Those steps 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 H3PO2

Phosphorous is a group VA element. 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 their respective number of atoms in the molecule. Below, That step is done.

  • Total valence electrons given by Phosphorous atom = 5

There are three oxygen atoms in H3PO2 molecule, Therefore

  • Total valence electrons given by oxygen atoms = 6 *2 = 12

There are three hydrogen atoms in H3PO2 molecule, Therefore

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

There are no charges in H3PO2 molecule.

  • Total valence electrons = 5 + 12 + 3 = 20

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, H3PO2 molecule, Total number of pairs of electrons are 10.

Center atom and sketch of H3PO2 molecule

There are some requirements to be the center atom. Having a high valence is a main requirement to be a center atom. For H3PO2 molecule, phosphorous has the highest valence than oxygen and hydrogen.

Because, hypophosphorous acid is a monobasic 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 H3PO2 molecule.

H3PO2 sketch to draw lewis structure

Mark lone pairs on atoms

After drawing the sketch, we should start to mark lone pairs on atoms.

  • There are already two P-O bonds, one O-H bond and two P-H bonds in the above sketch. Therefore only five (10-5 = 10) valence electrons pairs are remaining to draw the rest of the structure.
  • Start to mark remaining five 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 lone pairs on atoms in H3PO4 skech

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. Hypophosphorous 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 of atoms in H3PO2

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. Therefore, We should try to reduce charges on atoms if it is a possible. 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 as the following figure.
  • Now there is a double bond between phosphorous atom and one oxygen atom. Now, there is only a single P-O single bonds between phosphorous atom and other oxygen atom.
onvert lone pairs to bonds in H3PO2

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