H2CO3 Lewis Structure, Molecular Geometry, Hybridization, and MO Diagram


H2CO3, known as carbonic acid is a compound of carbon, hydrogen, and oxygen. It is a weak acid (with pH 4.18) formed when carbon dioxide (CO2) dissolves in water.

However, when carbon dioxide is water, only a little quantity of the gas is dissolved in water. It is a very unstable acid that remains in equilibrium as the solution disassociates soon to form H+ ion and HCO3- ion.

Carbonic acid is a diprotic acid from which two types of salts, hydrogen carbonate (HCO3) and carbonate (CO3) are formed. It is also known as a respiratory acid since we exhale this acid in gaseous form.

H2CO3 is capable to dissolve calcium carbonate (CaCO3). As a result, calcium bicarbonate salt is formed. When carbonic acid reacts with a small amount of base, bicarbonate salt is formed.

However, when it reacts with a large quantity, carbonate is formed. H2CO3 is obtained as a by-product of the combustion of fossil fuel in large quantities.

It is also obtained from fermentation in the industry. H2CO3 is a very important industrial chemical with a large number of uses.

It is used in preparing carbonated water, and other aerated drinks.

In the coming sections, we will have a look at H2CO3 Lewis Structure, Molecular Geometry, Hybridization, and MO Diagram.


Lewis Structure of Carbonic Acid (H2CO3)

The formula of carbonic acid is H2CO3. It has two H atoms, one C atom, and three O atoms.

To understand the molecular formula of H2CO3, we have to observe the electronic configuration of the participating atoms and how many atoms they have in the outer shell.

The electronic configuration of the atoms present in the H2CO3 molecule is as below:

Hydrogen → 1S2
Carbon → 1S2 2S2 2P2
Oxygen → 1S2 2S2 2P4

If you look at the electronic configuration of the elements, you will see hydrogen has a lonely electron, carbon has four electrons while oxygen has six electrons in the valence shell.

It will be helpful to know how these elements share electrons with each other, and form a chemical bond.

Let’s look at different steps to know the Lewis structure of H2CO3 now.

Step One: Find the no. of electrons in the outer orbital.

The two hydrogen atoms have two valence electrons, carbon has four, while three oxygen atoms have a total of 18 electrons.

Hence the total electrons in the outermost shell are 2+4+18 = 24

Step Two: Find the number of electrons required for the Octet.

Two hydrogen needs four electrons; while one carbon atom needs eight electrons and three oxygen atoms need 24 electrons.

So, the total number of electrons required for an octet is 2*2+8+3*8 = 36.

Step Three: Find the no. of electrons participating in bonding.

To find the number of bonding electrons, we have to subtract the no. of electrons in the outer orbit from the total no. of electrons required for an octet.

So, number of bonding electrons = 36-24 = 12.

Step Four: In this step, we will find how many bonds are there in the molecule.

In H2CO3, it is 12/2 = 6.

Step Five: In this step, first we need to find how many lone pairs (electrons not forming a bond) are there in the molecule.

To find it, we need to deduct the number of electrons taking part in the formation of bonds from the no. of electrons in the outermost shell. In the case of H2CO3, it is 24 – 12 = 12, that is six lone pairs.

Central Atom

Now we need to find the central atom first to draw the Lewis structure. Here it is carbon as it has the lowest electronegativity.

In the next step, we arrange the other atoms, and lastly, we have to draw the bond pairs and lone pairs.

H2CO3 Lewis Structure

In the Lewis structure of HNO3, carbon atom forms bonds with OH ions at the two sides while the third oxygen atom is placed on the third side.

The carbon atom attains a stable electronic configuration with eight electrons at the outermost shell. It forms two single bonds with two OH ions while a double bond with the remaining oxygen bond.

The two hydrogen atoms from single bonds in OH thus attaining an Octet. The oxygen atoms in OH form a single bond with hydrogen and a single bond with carbon.

So, they have two bonds and two lone pairs. The third oxygen atom forms a double bond with the carbon atom. It also has two lone pairs of electrons.


Molecular Geometry of H2CO3

The molecular geometry of a compound depends largely on two things; first, the Lewis structure, and the second is VSEPR (valence shell electron pair repulsion) theory.

When we look at the Lewis structure of H2CO3, we can see that H has one, C has four, and O has six electrons in the outer shell.

The geometrical shape of the H2CO3 molecule is trigonal planar.

The Carbon and 3 Oxygen atoms lie in the same plane.

During bond formation, the C atom connects with one O atom and two OH ions. The three oxygen atoms have two lone pairs each.

As per VSEPR theory, the electron pairs from different atoms or the bonds stay at the largest distance because of the repulsive force.

The strong repulsive force provides a stable force. Besides these factors, the structure of a molecule depends on the number of covalent bonds and lone pairs.

Another important factor that helps to find the molecular structure is the steric number (SN).

We can find it by adding the no. of lone pairs on the central C atom and the no. of atoms bonding with the central atom.

There is no lone pair on carbon and the number of atoms carbon is bonded with is 3. So, the steric number (S/N) is 3.

H2CO3 shape

Hybridization of H2CO3 Molecule

Hybridization is the process of combining two orbitals with the same energy levels to obtain a new one. To understand the shape of a molecule, it is essential to know the atoms are arranged in it.

The arrangement of atoms helps us know about the structure and properties of the substance.

Carbon has a steric number of 3, while the oxygen atom in the OH ion has a steric number of 4.

The carbon atom in H2CO3 has SP2 hybridization, and the O atom has SP3 hybridization.

After bonding, H2CO3 has SP2 hybridization.

Here is why, number of bond pair = (1*2 + 4 + 6*3) /8 = 3.

It means the molecule H2CO3 is SP2 hybridized.


MO Diagram of H2CO3

The sigma bonds between C and O atoms are formed by the 2sp2 orbital of C and O atoms. It results in sigma bonding and antibonding orbitals.

The sigma bond between oxygen and hydrogen use 1s orbital of hydrogen and 2sp3 orbitals of oxygen. It forms a sigma bonding and antibonding orbital.

The other 2sp2 orbital of oxygen is not involved in bonding.


Polarity of H2CO3

The term ‘polarity’ determines the property according to which the molecule exhibits the polarization of charges across the atoms of the molecule.

The polarity rises in a molecule when the shape of the molecule is asymmetric and there exists a difference of charge intensity on atoms.

Therefore the net dipole moment of the molecule results in some nonzero value.

Similarly,  in the H2CO3 molecule, high charge intensity on the oxygen atom results in polarization across the molecule.



Carbonic acid (H2CO3) is a very interesting compound that is widely used in different industries, especially in making beverages.

Studying its Lewis structure, molecular geometry, and hybridization will help you know a lot of things about this compound.

You can understand the structure, physical and chemical properties of the compound. We hope this information is useful to you.

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