CO32- is the chemical formula for a carbonate ion. It is the conjugate base of carbonic acid (H2CO3). It is widely used as a raw material in the industrial sector, especially in glass making, paper, and pulp industry, and for the production of soaps and detergents. CO32- has a molar mass of 60.008 g/mol.
So, what is the polarity of this widely used molecular ion in different industrial processes? Is carbonate (CO32-) ion polar or non-polar?
Let’s find out in this article.
Is CO32- polar or non-polar?
Carbonate (CO32-) is a non-polar molecular ion. It consists of one carbon (C) atom and three oxygen (O) atoms. The carbon atom is present at the center of the molecular ion, while all three oxygen atoms occupy the terminal positions, one on each side, making a symmetrical trigonal planar molecular shape and geometry.
An electronegativity difference of 0.89 units exists between a carbon and an oxygen atom in each of the two C-O bonds and one C=O bond.
Hence, all C-O and C=O bonds are individually polar and possess a specific dipole moment value (symbol µ).
However, it is due to the symmetrical shape of CO32- that the dipole moments of both C-O bonds and the C=O bond cancel out each other in the molecular ion.
The charged electron cloud stays uniformly distributed in the molecular ion overall. Thus, carbonate (CO32-) is a non-polar ion (net µ = 0).
A molecule is polar if there is a non-uniform charge distribution present in it. If the charge distribution gets equally balanced in different parts, then that molecule or molecular ion is considered non-polar.
The following three factors mainly influence the polarity of a molecule:
The electronegativity difference between two or more covalently bonded atoms
Dipole moment
Molecular geometry or shape
Now, let us discuss the effect of the above three factors one by one to prove that the carbonate (CO32-) is overall a non-polar molecular ion.
Factors affecting the polarity of CO32-
Electronegativity
It is defined as the ability of an atom to attract a shared pair of electrons from a covalent chemical bond.
Electronegativity decreases down the group in the Periodic Table of elements while it increases across a period.
Greater the electronegativity difference between bonded atoms in a molecule or molecular ion, the higher the bond polarity.
Carbon (C) is present in Group IV A of the Periodic Table. The electronic configuration of carbon is 1s2 2s2 2p2. As per this electronic configuration, a C-atom has a total of 4 valence electrons. It is thus short of 4 more electrons that are required so that the carbon atom can achieve a complete octet electronic configuration.
Conversely, oxygen (O) belongs to Group VI A of the Periodic Table. The electronic configuration of an oxygen atom is 1s2 2s2 2p4. So each O-atom has a deficiency of 2 more valence electrons for it to complete its octet.
Hence in the CO32- ion, the central carbon atom is double-covalently bonded to one oxygen atom and single-covalently bonded to each of the remaining two oxygen atoms. No lone pair of electrons is present on the central C-atom as all four of its valence electrons get consumed in covalent bonding.
However, among the terminal O-atoms, the double-bonded oxygen atom contains two lone pairs of electrons while each single covalently bonded oxygen atom contains three lone pairs of electrons.
In this way, all atoms in the molecular ion attain a complete octet electronic configuration in their respective shells via lone pairs of electrons and chemical bonding in the CO32- ion.
Due to the absence of any lone pair of electrons on the central carbon (C) atom, there is no distortion in the shape and geometry of the molecular ion.
The carbonate ion thus attains a symmetrical trigonal planar shape and geometry.
Atom
Electronic configuration
Valence electrons
Carbon (6C)
1s22s22p2
4
Oxygen (8O)
1s22s22p4
6
The electronegativity of carbon is 2.55 and that of oxygen is 3.44. The electronegativity difference between the carbon and oxygen atoms is thus 0.89 units.
Therefore, the oxygen atom present at the terminal positions of the molecular ion gains a partial negative (Oδ-) charge, while the central carbon atom obtains a partial positive (Cδ+) charge.
As a result, the C-O and C=O bonds are individually polar in CO32- ion.
Dipole Moment
Dipole moment (μ) is a vector quantity that points from the positive pole to the negative pole of a bond or a molecule.
It is mathematically calculated as a product of the magnitude of charge (Q) and charges separation (r). The dipole moment is expressed in a unit called Debye (D).
The dipole moment of a polar covalent bond conventionally points from the positive centre to the centre of the negative charge.
So inCO32-, the dipole moment of each of the C-O and C=O bonds points fromCδ+ to Oδ- (as shown below).
Molecular geometry
According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, CO32- is an AX3E0 or simply AX3-type molecular ion. To one C-atom (A), three (one C=O and two C-O) bond pairs (X) are attached, and there is no lone pair of electrons (E) present on the central carbon atom.
So the molecular geometry or shape of the CO32- ion is identical to its ideal electron pair geometry, i.e., trigonal planar. Due to the absence of any lone pair of electrons on the central C-atom, there are no lone pair-lone pairs or lone pair-bond pair electronic repulsions present in the molecular ion.
Thus, there is no distortion present in the shape and geometry of the carbonate ion. The O-C=O bonded atoms possess a mutual bond angle of 120°.
It is due to the symmetrical trigonal planar shape of the CO32- ion that the dipole moment of an upwards-pointing C=O bond gets uniformly canceled with the net dipole moment of two downwards-pointing C-O bonds.
Therefore, the charged electron cloud stays uniformly spread over the molecular ion. Consequently, carbonate CO32- is a non-polar molecular ion overall (net µ = 0).
Difference between polar and nonpolar?
Polar molecule
Non-polar molecule
Atoms must have a difference in
electronegativity
Atoms may have the same or different electronegativity values
Unequal charge distribution overall
Equal charge distribution overall
Net dipole moment greater than zero
Net dipole moment equals to zero
Examples include water (H2O), ethanol (CH3CH2OH), ammonia (NH3), sulfur dioxide (SO2), bromine trifluoride (BrF3), nitric oxide (NO), carbonyl sulfide (OCS or COS), etc.
Examples include oxygen (O2), nitrogen (N2), methane (CH4), ethane (C2H6), propane (C3H8), ethyne (C2H2), silicon dioxide (SiO2), carbonate ion (CO32-), etc.
Why is carbonate (CO32-) a non-polar molecular ion?
CO32- possesses two C-O bonds and one C=O bond. The electronegativity difference between the carbon (E.N = 2.55) and oxygen (E.N = 3.44) atoms is 0.89 units.
Each of the two C-O as well as the C=O bond, are thus individually polar and possess an identical dipole moment.
However, it is due to the symmetrical trigonal planar shape of CO32- that the dipole moments of individually polar bonds get uniformly canceled in opposite directions.
Consequently, carbonate (CO32-) is a non-polar molecular ion overall (net µ = 0).
How can an anion like CO32- be non-polar even if it has two polar single (C-O) and one polar double (C=O) bond?
The polarity of any molecule or ion depends upon the polarity of the individual bonds in the structure, as well as on the symmetry or asymmetry of the polar bonds.
In the carbonate CO32- ion, each of the carbon and oxygen bond is polar due to the electronegativity difference of 0.89 units between the bonded atoms.
However, because the three (two C-O, one C=O) bonds are arranged in a symmetrical manner around the central atom in the molecular ion, the carbonate CO32- is overall (net µ = 0) a non-polar molecular ion.
Why is CO32- insoluble in water while Na2CO3 is water-soluble?
CO32- has a symmetrical trigonal planar geometry, and the dipole moments of all three bonds (two C-O and one C=O) cancel each other out. The ion is thus non-polar and has net µ = 0. Like dissolves like, so the non-polar carbonate ion is insoluble in polar water molecules.
Similarly, many carbonate salts are water-insoluble, with a few exceptions. These exceptions include carbonate salts of sodium, potassium and ammonium, while magnesium carbonate is slightly water soluble.
When added to water, Na2CO3 breaks down into 2 Na+ and 1 CO32–ion per Na2CO3 unit cell, forming carbonic acid and sodium hydroxide and thus getting solubilized.
Why is the H3O+ ion polar while CO32- is non-polar?
The hydronium (H3O+) ion consists of three strongly polar O-H bonds with an electronegativity difference of 1.24 units between the bonded atoms. Each O-H bond thus possesses a high dipole moment value.
In contrast, CO32- consists of polar C-O bonds, but the individual dipole moments get uniformly canceled in the symmetrical trigonal planar shape of the molecular ion.
Thus, CO32- isoverall non-polar (net µ = 0).
Is there a formal charge on the bonded atoms in the CO32- molecule?
Formal charge of an atom = [ valence electrons – non-bonding electrons- ½ (bonding electrons)]
For the central C-atom
Valence electrons = 4
Bonding electrons = 8
Non-bonding electrons = 0
∴ Formal charge on the central C-atom = 4 – 0 – 8/2 = 4 – 0 – 4 = 4 – 4 = 0
Zero formal charges are present on the central C-atom and the double-bonded O-atom, while each single bonded O-atom carries a -1 formal charge in the CO32- ion.
Thus, the overall charge present on the carbonate CO32- ion is (0) + (0) + 2(-1) = -2.
Summary
Carbonate (CO32-) is a non-polar molecular ion.
It consists of polar bonds (one C=O and two C-O bonds) due to an electronegativity difference of 0.89 units between the bonded C-atom (E.N =2.55) and O-atom (E.N =3.44).
Carbonate CO32- ion has a symmetrical trigonal planar shape with a 120° mutual bond angle.
The charged electron cloud stays uniformly distributed in the symmetrical CO32- trigonal planar shape.
Due to the equal value of an upwards-pointing C=O dipole moment and the net dipole moment of two downwards-pointing C-O bonds, the individual dipole moments get canceled out uniformly.
The net dipole moment of CO32- is thus 0, so carbonate is overall a non-polar molecular ion.
Vishal Goyal is the founder of Topblogtenz, a comprehensive resource for students seeking guidance and support in their chemistry studies. He holds a degree in B.Tech (Chemical Engineering) and has four years of experience as a chemistry tutor. The team at Topblogtenz includes experts like experienced researchers, professors, and educators, with the goal of making complex subjects like chemistry accessible and understandable for all. A passion for sharing knowledge and a love for chemistry and science drives the team behind the website. Let's connect through LinkedIn: https://www.linkedin.com/in/vishal-goyal-2926a122b/
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