H3PO4 Lewis Structure in 6 Steps (With Images)

So you have seen the above image by now, right?

Let me explain the above image in short.

H3PO4 (phosphoric acid) lewis structure has a Phosphorus atom (P) at the center which is surrounded by one Oxygen atom (O) and three O-H groups. There is a double bond between the Phosphorus (P) & Oxygen (O) atom and a single bond between the Phosphorus (P) and three O-H groups.

If you haven’t understood anything from the above image of H3PO4 lewis structure, then just stick with me and you will get the detailed step by step explanation on drawing a lewis structure of H3PO4.

So let’s move to the steps of drawing the lewis structure of H3PO4.

Steps of drawing H3PO4 lewis structure

Step 1: Find the total valence electrons in H3PO4 molecule

In order to find the total valence electrons in H3PO4 (phosphoric acid) molecule, first of all you should know the valence electrons present in hydrogen atom, phosphorus atom as well as oxygen atom.
(Valence electrons are the electrons that are present in the outermost orbit of any atom.)

Here, I’ll tell you how you can easily find the valence electrons of hydrogen, phosphorus as well as oxygen using a periodic table.

Total valence electrons in H3PO4 molecule

→ Valence electrons given by hydrogen atom:

Hydrogen is group 1 element on the periodic table. ^[1] Hence the valence electron present in hydrogen is 1.

You can see that only 1 valence electron is present in the hydrogen atom as shown in the above image.

→ Valence electrons given by phosphorus atom:

Phosphorus is group 15 element on the periodic table. ^[2] Hence the valence electrons present in phosphorus is 5.

You can see the 5 valence electrons present in the phosphorus atom as shown in the above image.

→ Valence electrons given by oxygen atom:

Oxygen is group 16 element on the periodic table. ^[3] Hence the valence electrons present in oxygen is 6.

You can see the 6 valence electrons present in the oxygen atom as shown in the above image.

Hence, 

Total valence electrons in H3PO4 molecule = valence electrons given by 3 hydrogen atoms + valence electrons given by 1 phosphorus atom + valence electrons given by 4 oxygen atoms = 1(3) + 5 + 6(4) = 32.

Step 2: Select the central atom

For selecting the center atom, you have to remember that the atom which is less electronegative remains at the center.

(Remember: If hydrogen is present in the given molecule, then always put hydrogen outside.)

Now here the given molecule is H3PO4 and it contains hydrogen atoms (H), phosphorus atom (P) and oxygen atoms (O).

So as per the rule we have to keep hydrogen outside.

Now, you can see the electronegativity values of phosphorus atom (P) and oxygen atom (O) in the above periodic table.

If we compare the electronegativity values of phosphorus (P) and oxygen (O) then the phosphorus atom is less electronegative.

So here the phosphorus atom (P) is the center atom and the oxygen atom (O) is the outside atom.

Step 3: Connect each atoms by putting an electron pair between them

Now in the H3PO4 molecule, you have to put the electron pairs between the phosphorus (P) & oxygen (O) atoms and between the oxygen (O) & hydrogen (H) atom.

This indicates that these atoms are chemically bonded with each other in a H3PO4 molecule.

Step 4: Make the outer atoms stable

Now in this step, you have to check the stability of the outer atoms.

Here in the sketch of H3PO4 molecule, you can see that the outer atoms are hydrogen atoms and oxygen atoms.

These hydrogen atoms and oxygen atoms are forming a duplet and octet respectively and hence they are stable.

Also, in step 1 we have calculated the total number of valence electrons present in the H3PO4 molecule.

The H3PO4 molecule has a total 32 valence electrons and all these valence electrons are used in the above sketch of H3PO4.

Hence there are no remaining electron pairs to be kept on the central atom. 

So now let’s proceed to the next step.

Step 5: Check the octet on the central atom

In this step, you have to check whether the central phosphorus atom (P) is stable or not.

In order to check the stability of the central phosphorus (P) atom, we have to check whether it is forming an octet or not.

You can see from the above picture that the phosphorus atom is forming an octet. That means it has 8 electrons.

And hence the central phosphorus atom is stable.

Now let’s proceed to the final step to check whether the lewis structure of H3PO4 is stable or not.

Step 6: Check the stability of lewis structure

Now you have come to the final step in which you have to check the stability of lewis structure of H3PO4.

The stability of lewis structure can be checked by using a concept of formal charge.

In short, now you have to find the formal charge on hydrogen (H) atoms, phosphorus (P) atom as well as oxygen (O) atoms present in the H3PO4 molecule.

For calculating the formal charge, you have to use the following formula;

Formal charge = Valence electrons – (Bonding electrons)/2 – Nonbonding electrons

You can see the number of bonding electrons and nonbonding electrons for each atom of H3PO4 molecule in the image given below.

For Hydrogen (H) atom:
Valence electron = 1 (because hydrogen is in group 1)
Bonding electrons = 2
Nonbonding electrons = 0

For Phosphorus (P) atom:
Valence electrons = 5 (because phosphorus is in group 15)
Bonding electrons = 8
Nonbonding electrons = 0

For Oxygen (O) atom:
Valence electrons = 6 (because oxygen is in group 16)
Bonding electrons = 2
Nonbonding electrons = 6

For Oxygen (O) atom (of O-H group):
Valence electrons = 6 (because oxygen is in group 16)
Bonding electrons = 4
Nonbonding electrons = 4

From the above calculations of formal charge, you can see that the phosphorus (P) atom has +1 charge while the oxygen atom has -1 charge.

So let’s keep these charges on the respective atoms of the H3PO4 molecule.

The above lewis structure of H3PO4 is not stable. So we have to minimize these charges by shifting the electron pairs from the oxygen atom to the phosphorus atom.

After shifting the electron pair from oxygen atom to phosphorus atom, the lewis structure of H3PO4 becomes more stable.

In the above lewis dot structure of H3PO4, you can also represent each bonding electron pair (:) as a single bond (|). By doing so, you will get the following lewis structure of H3PO4.

I hope you have completely understood all the above steps.

For more practice and better understanding, you can try other lewis structures listed below.

Try (or at least See) these lewis structures for better understanding:

About author

Jay Rana

Jay is an educator and has helped more than 100,000 students in their studies by providing simple and easy explanations on different science-related topics. He is a founder of Pediabay and is passionate about helping students through his easily digestible explanations.

Read more about our Editorial process.