SO32- is the chemical formula for the sulfite ion that has a molar mass of 80.07 g/mol. Sulfites are naturally found in the human body, and in food items such as peanuts, eggs, and black tea.
The sulfite salts have been a lifesaver in food and beverage industries, such as they prevent fruits from browning and wine from turning into vinegar.
This article will discuss a very important property of the sulfite ion SO32- i.e., its polarity.
To know further, continue reading this article.
Is SO32- polar or non-polar?
Sulfite (SO32-) is a polar molecular ion. It consists of one sulfur (S) atom and three oxygen (O) atoms. The S-atom is present at the center of the molecular ion while 3 O-atoms occupy terminal positions, one on each side, making one S=O and two S-O bonds adopting a trigonal pyramidal molecular geometry.
An electronegativity difference of 0.86 units exists between a sulfur and an oxygen atom.
Hence, each S-O and S=O bond is individually polar in SO32- and possesses a specific dipole moment value as well (symbol µ).
However, it is due to the asymmetrical shape of the SO32- ion that the charged electron cloud stays non-uniformly distributed, and the dipole moments of individually polar S=O and S-O bonds do not get canceled equally.
Consequently, SO32- is overall a polar molecular ion (net µ > 0).
A molecule or molecular ion 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 sulfite (SO32-) ion is overall polar.
Factors affecting the polarity of SO32-
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.
Sulfur (S) belongs to Group VI A (or 16) of the Periodic Table. The electronic configuration of a sulfur atom is 1s22s22p63s23p4. Hence, each S-atom in SO32- ion lacks 2 valence electrons to obtain a complete octet electronic configuration.
Conversely, oxygen (O) also belongs to Group VI A (or 16) of the Periodic Table. The electronic configuration of an oxygen atom is 1s2 2s2 2p4. This implies that each O-atom has a deficiency of 2 more valence electrons for it to complete its octet.
Hence, in SO32-, the central sulfur atom is single-covalently bonded to two oxygen atoms and double-covalently bonded to one oxygen atom.
4 valence electrons of the central S-atom consumed in covalent bonding out of the 6 initially available leaves behind 2 valence electrons that are situated as a lone pair on the S-atom in SO32-
Contrarily, there are 3 and 2 lone pairs on each of the single and double-covalently bonded O-atoms, respectively, as shown in the SO32- Lewis dot structure drawn below.
Atom
Electronic configuration
Valence electrons
Sulfur (16S)
1s22s22p63s23p4
6
Oxygen (8O)
1s22s22p4
6
An electronegativity difference of 0.86 units exists between the bonded S-atom (E.N = 2.58) and O-atom (E.N = 3.44) in S=O and each S-O bond.
Therefore, in the SO32- ion, each of the terminal oxygen atoms gains a partial negative (Oδ-) charge while the central sulfur atom obtains a partial positive (Sδ+) charge.
As a result, each S-O and S=O bond is individually polar in the sulfite SO32- 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 charge 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 center to the center of the negative charge.
So, in SO32-, the dipole moment of each S-O and S=O bond point fromSδ+ to Oδ- (as shown below).
Molecular geometry
According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, SO32- is an AX3E1-type molecular ion. To one S-atom at the center (A), three (two S-O and one S=O) bond pairs (X) are attached, and there is one lone pair of electrons (E) on the central atom.
So, the molecular geometry or shape of the SO32- ion is trigonal pyramidal, while its ideal electron pair geometry is tetrahedral.
Due to the presence of 1 lone pair of electrons on the central S-atom in SO32-, strong lone pair-bond pair electronic repulsions exist in the sulfite ion, in addition to a bond pair-bond pair repulsive effect.
To minimize the repulsive effect, the molecular geometry gets distorted, and the O=S-O bond angle becomes 106°, different from an ideal bond angle of 109.5° in a tetrahedral shape.
In the asymmetric trigonal pyramidal shape of the molecular ion, the dipole moments of all individually polar (two S-O and one S=O) bonds do not get canceled equally in opposite directions.
The charged electron cloud does not stay uniformly distributed.
Consequently, sulfite (SO32-) ion is overall a polar molecular ion (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), sulfite (SO32-), etc.
Examples include oxygen (O2), nitrogen (N2), methane (CH4), ethane (C2H6), propane (C3H8), ethyne (C2H2), silicon dioxide (SiO2), carbonate ion (CO32-), sulfate (SO42-), etc.
The sulfite (SO32-) ion has polar bonds (two S-O and one S=O) due to an electronegativity difference of 0.86 units between the bonded S-atom (E.N = 2.58) and O-atom (E.N = 3.44).
The central S-atom contains one lone pair of electrons. Therefore, SO32- attains an asymmetrical trigonal pyramidal shape, due to which the dipole moment of all individually polar (two S-O and one S=O) bonds do not get canceled uniformly.
Consequently, the sulfite SO32- ion is overall a polar molecular ion with a permanent dipole moment value.
Sulfur trioxide SO3 and sulfite SO32- have similar and the same number of atoms, but still they differ in their polarity. Why?
SO32- has an asymmetricaltrigonal pyramidal shape due to the presence of one lone pair of electrons on the central S-atom.
The dipole moments of individually polar two S-O and one S=O bond do not get canceled equally.
The charged electron cloud stays non-uniformly spread over the molecular ion. So it is overall polar (net µ > 0).
Contrarily, SO3 has a symmetrical trigonal planar shape. Due to the absence of any lone pair of electrons on the central S-atom, its shape and molecular geometry is symmetrical in which the charged electron cloud is uniformly distributed.
The dipole moments of individually polar (three S=O) bonds get canceled equally in opposite directions.
Consequently, sulfite SO32- is a polar ion with net dipole moment μ > 0, while sulfur trioxide SO3 is a non-polar molecule with net μ = 0.
Why is SO32- polar while CO32- is a non-polar molecular ion?
SO32- has an asymmetrical trigonal pyramidal. The central S-atom contains onelone pair of electrons, due to which the charged electron cloud does not stay uniformly distributed, and the dipole moment of individually polar (two S-O and one S=O) bonds do not get canceled equally.
∴ Formal charge on each single bonded O-atom = 6 – 6 – 2/2 = 6 – 6 – 1 = 6 – 7 = -1
Zero formal charges are present on the central S-atom and on the double-bonded O-atom. While a -1 formal charge is present on each of the two single covalently bonded oxygen atoms.
Hence, the overall charge present on the sulfite SO32- ion is (0) + (0) + 2(-1) = -2.
Summary
Sulfite (SO32-) is a polar molecular ion.
It consists of polar bonds, including one S=O and two S-O bonds, due to an electronegativity difference of 0.86 units between the bonded S-atom (E.N = 2.58) and O-atom (E.N = 3.44).
The central S-atom contains one lone pair of electrons, due to which the sulfite (SO32-) ion has an asymmetrical trigonal pyramidal molecular shape while a tetrahedral electron pair geometry.
The O=S-O bond angle is equal to 106°.
The charged electron cloud stays non-uniformly distributed in the asymmetrical trigonal pyramidal shape of SO32-.
The dipole moments of all individually polar (two S-O and one S=O) bonds do not get canceled in opposite directions. The net dipole moment in sulfite SO32- is non-zero (net µ > 0) hence it is overall a 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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