Carbonates are one of the most commonly found and discussed ionic entities in the field of chemistry. Salt of the carbonic acid, carbonates are widely used in a variety of industrial and domestic applications. Some of these include glass and ceramic creation, food preservation, and iron extraction.
CO32- ion is the simplest oxocarbon anion that decomposes on heating and is usually water-insoluble barring a few exceptions.
Let us now study the chemical bonding of the CO32- ion in detail.
CO32- Lewis Structure
If you are reading this article, you have probably already come across this term, haven’t you?
It is needless to mention when one learns about the nature of chemical bonding across atoms and molecules, Lewis Structure is a concept that we cannot simply factor out.
To get a quick and clear overview of the atomic bonding across elements, all we need to do is to first sketch a 2D diagrammatic representation of the given molecule. Lewis Structure is the name given to such a skeletal diagram where we use the symbols of the atoms and use dots to represent the valence shell electrons.
Hence, Lewis Structure is also commonly called Electron Dot Structure.
Let us proceed to draw the most appropriate LS diagram of CO32- ion.
Step 1: Count the Total Number of Valence Electrons.
In CO32- ion, we have one carbon atom and three oxygen atoms along with two negatively charged electrons carrying the charge.
Valence electrons refer to the number of electrons in the outermost shell of an atom around the nucleus that help in determining the valency of the given atom. We can easily find out the value from the atomic number mentioned in the periodic table.
In the case of the carbonate ion,
Carbon has an atomic number of 6 and a valency of 4. Oxygen has an atomic number of 8 and a valency of 6.
total valence electron number in CO32- is
= 4 + 6*3 + 2
Step 2: Determine the Central Atom of the Molecule.
Now, in order to draw the Lewis Structure, we have to determine which one is the central atom in a multiatomic heterogeneous molecule, here an ion.
In carbonate ion, among the two elements, Carbon has an electronegativity value of 2.55 whereas Oxygen has a high value of 3.44.
As per common procedure, the one with the least electronegativity value will work as the central atom.
Carbon is the central atom here.
Step 3: Draw the Skeletal Diagram of the Molecule.
With the help of dots for valence electrons and atomic symbols for the elements, we will be able to draw the primary sketch of the carbonate ion.
For this, we will first have to incorporate the octet rule.
The elements present in the main group usually tend to follow the concept of octet fulfillment. This means that these atomic elements will incline towards having eight valence electrons just like the noble gas configurations of the same period.
Let us draw the skeletal diagram for CO32- ion:
Step 4: Bond formation
Here, as we can see, we have drawn the sketch.
- The total number of valence electrons has been made equal to 24.
- Two electrons have been shared between carbon and each of the three oxygen atoms which point towards the existence of single bonds.
- The octet rule has been fulfilled for all the oxygen atoms.
- For the Carbon atom, however, we have only six electrons hence the octet configuration has not been satisfied.
We can consider having a double bond between any one of the oxygen atoms and carbon which will lead to carbon having eight valence electrons around itself.
Look at this Lewis Structure.
We have each of the oxygen having octet configuration,
We have carbon having octet configuration.
We have two single bonds between carbon and each of the two oxygen atoms.
We have one double bond between C and one O atom.
After drawing bonds, the lewis structure looks like this :
It can also be shown in the below image.
Step 5: Formal Charge
To check whether the above-mentioned sketch is the best possible Lewis Structure of Carbonate ( CO32-) ion, we will have to check the formal charge values.
Sometimes, we assign a charge to a bonded atom with the assumption that the charge is shared equally among all the bonded atoms. This is known as the formal charge.
The formula for formal charge:
Let us find out for CO32- :
For Carbon, formal charge= 4 – 0.5*8 – 0 = 4 – 4 = 0.
For each of the O in a single bond with carbon, formal charge
= 6 – 0.5*2 – 6 = 6 – 1 – 6 = -1.
For the O atom in a double bond with carbon, formal charge
= 6 – 0.5*4 – 4 = 6 – 2 – 4 = 0.
We can see that the formal charge values of every atom are maintained at their lowest possible forms. Therefore, our Lewis Structure has been completed.
CO32- Molecular Geometry
Is a 2D structure sufficient enough for getting an in-depth understanding of the bonding happening inside a molecule?
Well, a perfectly drawn Lewis Structure does introduce to us the basic representation of constituent atoms inside any molecule or ion and also talks about the type of bonds formed.
But this is not enough.
Here comes the VSEPR theory or Valence Shell Electron Pair Repulsion Theory model which deals with determining the 3D nature of any molecular composition.
This is known as molecular geometry which mentions not only the shape of the molecule but also the bond lengths and the angles.
This helps us picture the molecule in a better and clearer manner.
Here, we can follow the AXn notation to find out the exact molecular geometry of CO32- ion.
This is known as VSEPR notation. In this theory, we talk about the minimum repulsion happening between negatively charged electron clouds to have a balanced molecular composition.
In the AXn notation,
A stands for the central atom,
X stands for the number of atoms surrounding the central atom,
n stands for the number of bonds attached to the central atom inside the molecule,
Ex stands for the number of lone pairs (non-bonded electron pairs) of the central atom.
For carbonate ion,
A= Carbon atom,
X= Oxygen atom,
Therefore the required notation is AX3.
Let us look at the chart provided above. As per the VSEPR chart with AXnEx notations, we can predict our molecular shape.
The shape of AX3 notation as in CO32- ion is trigonal planar with a bond angle of about 120 degrees.
If you are a student of chemistry, it is safe to assume that you are aware of the difference between an orbit and an orbital.
While orbit talks about the definite path of an electron around the atomic nuclei, orbital deals with the probability of electrons being present in any given space.
Atomic orbitals are of different shapes like spherical and dumb-bell shapes to name a few. Accordingly, they are called s,p,d,f.
Now, when chemical bonding occurs, these AOs come together and combine to form hybridized orbitals that take part in the formation of bonds inside a molecule.
This process is named hybridization.
If we have a look at the formal charge concept in Lewis Structure again, we can see that the singly bonded O atoms in C-O have a negative charge of -1 attached to each of them. However, the doubly bonded O atom in C=O has no charge value.
The single bonds signify the presence of sigma bonds whereas the double bond indicates the presence of both sigma and pi bonds.
Therefore we have 3 sigmas and 1 pi present here around the central C atom in carbonate ion. Below is the formula to quickly decipher the H ( Hybridization value) of an atom inside a molecule.
Here, V = 4, M = 0, C = 0, A = 2.
H = 0.5 ( 4 + 0 – 0 + 2 ) = 3.
For the value of 3 electron pairs, we have sp2 hybridization.
CO32- Molecular Orbital (MO) Diagram
What is MO theory?
Molecular Orbital Theory is a concept of quantum mechanics that is used to decipher the chemical bonding nature inside different molecular structures.
This is a complex yet useful tool that helps in sketching MO diagrams for better understanding. This theory treats electrons to be having both particle and wave-like nature.
Here, unlike valence bond theory where AOs from the same atom can only fuse to form hybridized orbitals resulting in hybridization, we can consider valence electrons to be shared among all atoms.
Therefore, AO s from different atoms can come together for fusion to form Molecular Orbitals ( MOs ).
In carbonate ion, molecular orbital theory can be best explained via delocalized pi bonding. A delocalized pi bond signifies that the electrons are free to have movement over multiple nuclei i.e. the pi ( π ) can appear in several conformations.
During drawing of the Lewis Structure, we have found out that there are 24 valence electrons. Each oxygen atom inside the ion has four non-bonding electrons.
We have 3 sigma bonds, therefore 6 used.
Carbon atomic number: 6
Oxygen atomic number: 8
Electronic configuration of C: 1s2 2s2 2p2.
Electronic configuration of O: 1s2 2s2 2p4.
The 2pz orbitals of carbon and three O atoms are available for delocalized pi bonding. We have two electrons filling bonding molecular orbital, four filling non-bonding MOs.
Therefore, the six pi electrons available are used up to occupy the lowest energy MOs – the bonding MOs. This is an example of four four-center π MO treatment.
Here, in this detailed article, we have done an extensive discussion on the chemical bonding nature of the famous carbonate anion.
We have covered the formation of Lewis Structure, deciphered the perfect molecular geometry and bond angles of 3D CO32-. Not only this, but we have also tackled orbital hybridization and quantum MO theory.