Phosphoric acid
(H3PO4) lewis structure is drawn using the concept of total valence electrons.
H3PO4 contains three elements phosphorous, oxygen and hydrogen. These elements
provide electrons (not all electrons) of last shell to form bonds. In this tutorial, we will learn,
how to construct the lewis structure of H3PO4 molecule.
written: Heshan Nipuna, updated: 25/05/2020
Lewis Structure of phosphoric acid molecule
Now, we are going to learn, how to draw this lewis structure of H3PO4 molecule step by step.
There are several steps to draw H3PO4 lewis structure and they are explained
in detail in this tutorial.
Find total number of electrons of the valance shells of phosphorous atom,
oxygen atom and hydrogen atoms.
Total electrons pairs in valence shells
Determine center atom from phosphorous atom,
oxygen atom and hydrogen atoms.
Put lone pairs on atoms and mark charges on atoms
Stability of lewis structure - Check the stability and minimize charges on atoms by
converting lone pairs to bonds to obtain the best lewis structure.
Total number of electrons of the valance shells of phosphorous atom, oxygen atom and hydrogen atoms.
There are three types of atoms; phosphorous, oxygen and hydrogen. These atoms are located at different groups and
their valence electrons exist in last shells are different.
Hydrogen is located at group 1 and has only one electron in its valence shell. Oxygen is a group 6 element
and contains six electrons in its valence shell. Other atom, phosphorous is a group 16 element and has
five electrons in its valence shell.
Also, when you determine the number of electrons of the valance shells, you should consider, how many atoms there are. As an example, there are three hydrogen atoms in mol dm-3.
Total valence electrons given by hydrogen atom = 1*3 = 3
Total valence electrons given by oxygen atom = 6*4 = 24
Total valence electrons given by phosphorous atom = 5*1 = 5
Total valence electrons = 3 + 24 + 5 = 32
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,
H3PO4, there are 32 valence electrons, so total pairs of electrons are 16.
Center atom of H3PO4
For being the center atom, ability of having greater valance is a reason. If we consider each element, maximum
valence of hydrogen, oxygen and phosphorous are 1, 2 and 5 respectively. By the experience of drawing lewis
structures, we know phosphorous has the greatest chance to be the center atom.
Sketch of H3PO4
Oxygen atoms are located around the carbon atom and hydrogen atoms have connected to oxygen atoms.
Lone pairs on atoms
There are four (4) bonds around the center atom (phosphorous) in the sketch of phosphoric acid molecule.
Also, there are three (3) O-H bonds in the sketch. Therefore total bonds in the sketch is seven (7).
Now, nine (16-7=9) electrons pairs are remaining to mark lone pairs.
As the next step, mark those six valence electrons pairs as lone pairs on outside atoms (oxygen and hydrogen atoms)
as lone pairs.
We know, hydrogen cannot keep more than two electrons in its valence shell. Therefore,
hydrogen will not get any remaining lone pair.
Each oxygen atom which have joint with a hydrogen atom will take two lone pairs. Like that three oxygen atom will
take six (6) lone pairs.
Now, three (9-6) lone pairs are remaining and they are marked on other oxygen atom.
Now, all lone pairs are finished and no lone pairs to mark on phosphorous atom.
Charges of atoms in the structure
There is -1 charge on one oxygen atom and there is +1 charge on phosphorous atom. There are no charges on other atoms.
Check the stability of drawn structure of H3PO4 and reduce charges on atoms by converting
lone pairs to bonds
If possible, you should try to reduce charges on atoms. You can see, that oxygen atom which has -1 charge has three
lone pairs can convert its one lone pair to make a bond with phosphorous atom.
Now there is a double bond between one oxygen atom and phosphorous atom. And there are no charge in any atom.
Therefore, this structure should be the lewis structure of H3PO4.
Structure of H3PO4, shape of molecule around phosphorous atom?
There are four σ bonds and no lone pairs around phosphorous atom in the lewis structure of H3PO4 molecule. Therefore total number of VSPER pairs are four. Therefore, shape of molecule around phosphorous atom is tetrahedral.
Questions asked by students.
which is the lewis structure for h3po4?
In the lewis structure of H3PO4, there is one double between phosphorous atom and one oxygeen atom. Rest of all bonds are single bonds. Also, there are no lone pairs on phosphorous atom. There are three O-H bonds which give some acidic charcteristics to H3PO4.
Can I know oxidation number of phosphorous atom of lewis structure of H3PO4
Yes. Oxidation number of phosphorous atom can be calculated from lewis structure of H3PO4.
What is correct about from following statements about the lewis structure of phosphoric acid?
Lewis structure of phosphoric acid contains -1 charge on one oxygen atom and +1 charge on phosphorous atom.
There are three single bonds in the molecule.
There are no charges at any atom.
One hydrogen atom is joint to phosphorous atom and remaining hydrogen atoms are joint to two oxygen atoms.
Statement 3 is correct.There is no charges at any atom in the lewis structure of phosphoric acid.
how do I obtain lewis structure of phosphoric acid from phosphate ion lewis structure?
phosphate ion lewis structure containes four oxygen atom. From those four oxygen atoms, there are three oxygen atoms each of them has -1 charge. Three hyrogen atoms are joint to those oxygen atoms to make lewis structure of phosphoric acid.
