Sulfite [SO3]2- ion Lewis dot structure, molecular geometry or shape, electron geometry, bond angle, hybridization, formal charge, polar vs non-polar
[SO3]2- is the chemical formula for sulfite, a molecular ion. The sulfite [SO3]2- ion is the anionic constituent of the salts of sulfurous acid (H2SO3) which are naturally present in some foodstuffs as well as in the human body. Let us explain to you some important facts about the sulfite ion through this article.
In this article, you will learn how to draw the Lewis dot structure of [SO3]2-, what is its molecular geometry or shape, electron geometry, bond angle, hybridization, formal charges, and whether the [SO3]2- ion is polar or non-polar in nature. All this information will help you understand the chemical nature of the sulfite [SO3]2- ion better.
So without any further delay, let’s start reading!
The Lewis structure of sulfite [SO3]2- ion is made up of a sulfur (S) atom and three oxygen (O) atoms. The sulfur (S) is present at the center of the molecular ion while oxygen (O) occupies the terminals, one on each side.
There are a total of 4 electron density regions around the central S atom in the Lewis structure of [SO3]2-. Out of these 4 electron density regions, there are 3 bond pairs and 1 lone pair of electrons which is situated on the central sulfur atom.
Drawing the Lewis structure of sulfite SO32- is not that difficult. You just need to grab a paper and pencil and follow the instructions given below and you will learn to draw the Lewis dot structure of sulfite within no time.
Steps for drawing the Lewis dot structure of SO32-
1. Count the total valence electrons in SO32-
The Lewis dot structure of a molecule is referred to as a simplified representation of all the valence electrons present in it. Therefore, the very first step while drawing the Lewis structure of SO32– is to count the total valence electrons present in the concerned elemental atoms.
There are two different elemental atoms present in the sulfite [SO3]2- ion, i.e., sulfur (S) atom and oxygen (O) atoms. When you look through the Periodic Table of elements, you will find that both sulfur (S) and oxygen (O) are located in Group VI A. So both sulfur and oxygen contain a total of 6 valence electrons in each atom.
The [SO3]2- ion consists of 1 S-atom and 3 O-atoms. Thus, the valence electrons in the Lewis dot structure of [SO3]2- = 1(6) + 3(6) = 24 valence electrons.
However, the twist here is that the [SO3]2- ion carries a negative (-2) charge which means 2 extra valence electrons are added in this Lewis structure.
∴ Hence, the total valence electrons available for drawing the Lewis dot structure of [SO3]2- = 24+2 = 26 valence electrons.
2. Choose the central atom
Electronegativity is defined as the ability of an atom to attract a shared pair of electrons from a covalent chemical bond. So, the atom which is least electronegative or most electropositive is placed at the centre of a Lewis structure. This is because this atom is most likely to share its electrons with the more electronegative atoms surrounding it.
As Sulfur (S) is less electronegative than oxygen (O) so, an S-atom is placed at the center of the [SO3]2- Lewis structure while the three O-atoms are placed in their surroundings, as shown below.
3. Connect outer atoms with the central atom
At this step of drawing the Lewis structure of a molecule or molecular ion, we need to connect the outer atoms with the central atom using single straight lines. As the O-atoms are the outer atoms in the Lewis structure of the sulfite [SO3]2- ion so all 3 oxygen atoms are joined to the central S-atom using straight lines, as shown below.
Each straight line represents a single covalent bond containing 2 electrons.
Now, if we count the total valence electrons used till this step out of the 26 available initially, there are a total of 3 single bonds in the structure above. Thus, 3(2) = 6 valence electrons are used till step 3.
Total valence electrons available – electrons used till step 3 = 26-6 = 20 valence electrons.
This means we still have 20 valence electrons to be accommodated in the Lewis dot structure of [SO3]2-.
4. Complete the octet of outer atoms
There are three O-atoms present as outer atoms in the Lewis structure of [SO3]2-. Each O-atom needs a total of 8 valence electrons in order to achieve a stable octet electronic configuration.
Each S-O bond already represents 2 electrons therefore all the three O-atoms require 6 more electrons each to complete their octet. Thus, these 6 valence electrons are placed as 3 lone pairs on each O-atom, as shown below.
5. Complete the octet of the central atom
Total valence electrons used till step 4 = 3 single bonds + 3 (electrons placed around O-atom, shown as dots) = 3(2) +3(6) = 24 valence electrons.
Total valence electrons available – electrons used till step 4 = 26-24 = 2 valence electrons.
So, these 2 valence electrons are now placed as a lone pair on the central sulfur atom, as shown below.
The structure above shows that there are a total of 3 single bonds and 1 lone pair on the central S-atom which makes a total of 8 valence electrons in its surroundings. Thus, the central S atom as well as all the three O atoms now have a complete octet electronic configuration.
But, the thing is, is this Lewis structure stable? Let’s check that using the formal charge concept.
6. Check the stability of the SO32- Lewis structure using the formal charge concept
The less the formal charge on the atoms of a molecule, the better the stability of its Lewis structure.
The formal charge can be calculated using the formula given below.
This calculation shows that a +1 formal charge is present on the central Sulfur atom and a -1 formal charge is present on each of the three oxygen (O) atoms.
But as we already mentioned; the fewer the formal charges on the bonded atom, the greater the stability of a Lewis structure.
Thus, we can reduce the formal charges present on the S and O-atoms by converting the lone pair of electrons present on any one terminal O-atom into a covalent bond between the central S atom and the terminal O-atom.
Let us see how that’s done.
7. Minimize the formal charges on atoms by converting lone pairs into covalent bonds
One lone pair from any one terminal oxygen atom is converted into a covalent bond between the central S-atom and the respective O-atom as shown below.
Now there are a total of 2 single bonds and 1 double bond around the central S-atom. We can again check the stability of this Lewis structure using the formal charge formula.
For sulfur atom
Valence electrons of sulfur = 6
Bonding electrons = 1 double bond + 2 single bonds = 1(4) + 2(2) = 8 electrons
So here, you can see that the formal charges present on the central sulfur (S) atom and a terminal oxygen (O) atom are reduced to zero. However, there is a -1 formal charge on each of the other two oxygen atoms.
-1 + (-1) = -2 which accounts for an overall negative 2 charge on the sulfite [SO3]2- ion. This ensures that it is a correct and stable Lewis representation for the sulfite [SO3]2- ion. The [SO3]2- Lewis structure is enclosed in square brackets and a negative 2 charge is placed at the top right corner, as shown below.
If you are worried about the extra 2 electrons in the sulfur atom which means more than the octet number of electrons. Then, that’s not a problem because sulfur (S) has an expanded octet. It can accommodate more than 8 valence electrons during chemical bond formation due to the presence of a 3d subshell in its atomic structure.
Another interesting fact to keep in mind is that the actual structure of a sulfite [SO3]2- ion is a hybrid of the following resonance structures. Each resonance structure is a way of representing the Lewis structure of a molecule or an ion.
These resonance structures show that the formal charges present on [SO3]2- atoms are not stationary, rather they keep moving from one position to another. Similarly, a double bond can be formed between the central sulfur and any one terminal oxygen atom out of all the three available.
In conclusion, all the above resonance structures contribute equally to the resonance hybrid which is the best possible Lewis structure of the sulfite [SO3]2- ion.
Now, that we have discussed everything about the Lewis structure of [SO3]2-, let us proceed forward to the next section of the article.
What are the electron and molecular geometry of SO32-?
The sulfite [SO3]2- ion has a trigonal pyramidal shape and molecular geometry while the ideal electron pair geometry of the ion is tetrahedral. There is a lone pair of electrons present on the central S-atom in [SO3]2-.
The lone pair-bond pair electronic repulsions distort the geometry and shape of the molecular ion and make it adopt a different shape from its electronic geometry.
Molecular geometry of [SO3]2-
The sulfite [SO3]2- ion has a trigonal pyramidal molecular geometry and shape. There is a lone pair of electrons present on the central sulfur atom in sulfite. Therefore, lone pair-bond pair electronic repulsions exist in the molecule in addition to bond pair-bond pair repulsions.
This repulsive effect distorts the shape of the molecule and makes it adopt a trigonal pyramidal shape. The lone pair is situated at the apex of a pyramid that has a triangular base, as shown below.
An important point to note is that the molecular geometry or shape of a molecule or a molecular ion strongly depends on the different number of bond pairs and lone pairs present on the central atom.
However, the ideal electron pair geometry only depends on the total number of electron pairs present on the central atom, no matter whether it’s a bond pair or a lone pair.
Let us see how this concept is applied to the sulfite [SO3]2- ion.
Electron geometry of [SO3]2-
According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, the ideal electron geometry of a molecule or a molecular ion containing a total of 4 electron density regions around the central atom is tetrahedral.
In the sulfite [SO3]2- ion, there are 2 single bonds, 1 double bond and 1 lone pair on the central sulfur atom which makes a total of 4 electron density regions. Thus it has a tetrahedral electron pair geometry.
An easy way to find the shape and geometry of the molecule is to use the AXN method.
AXN is a simple formula to represent the number of atoms bonded to the central atom in a molecule and the number of lone pairs present on it.
It is used to predict the geometry or shape of a molecule using the VSEPR concept.
AXN notation for [SO3]2- molecular ion
A in the AXN formula represents the central atom. In the [SO3]2- ion, sulfur is present at the center so A = Sulfur.
X denotes the atoms bonded to the central atom. In [SO3]2-, three oxygen (O) atoms are bonded to the central S so X=3.
N stands for the lone pairs present on the central atom. As per the Lewis structure of [SO3]2- there is one lone pair on central sulfur so N=1.
So, the AXN generic formula for the [SO3]2- ion is AX3N.
Now, you may have a look at the VSEPR chart below.
According to the VSEPR chart given above, the ideal electron geometry of a molecule or a molecular ion having AX3N generic formula is tetrahedral while its molecular geometry or shape is trigonal pyramidal, as we already noted down for the [SO3]2- ion.
Hybridization of [SO3]2-
The sulfite [SO3]2- ion has sp3 hybridization.
The electronic configuration of a sulfur (S) atom is 1s2 2s2 2p6 3s2 3p4.
The electronic configuration of oxygen (O) atom is 1s2 2s2 2p4.
During chemical bonding, the paired 3p electrons of sulfur get unpaired and one of these electrons shifts to an empty 3d atomic orbital of sulfur. Consequently, the 3s orbital hybridizes with three 3p orbitals to yield four sp3 hybrid orbitals. Each sp3 hybrid orbital has a 25 % s-character and a 75% p-character.
Three of the four sp3 hybrid orbitals contain a single electron each while the fourth sp3 hybrid orbital contains paired electrons. These paired electrons are situated as a lone pair on the central S atom in the sulfite [SO3]2- ion.
The two sp3 hybrid orbitals containing single electrons form S-O sigma (σ) bonds with the p orbitals of oxygen by sp3-p overlap, on each side of the [SO3]2- ion.
The remaining sp3 hybrid orbital of sulfur forms a sigma (σ) bond with an sp2 hybrid orbital of oxygen in the S=O double bond.
The unhybridized d orbital of sulfur overlaps with the unhybridized p orbital of this oxygen to form the required pi (π) bond in the S=O double bond of the sulfite ion.
Refer to the figure given below for understanding these concepts more clearly.
A shortcut to finding the hybridization present in a molecule or a molecular ion is by using its steric number against the table given below. The steric number of central S in [SO3]2- is 4 so it has sp3 hybridization.
Steric number
Hybridization
2
sp
3
sp2
4
sp3
5
sp3d
6
sp3d2
The steric number of central Sulfur in SO32- is 4 so it has sp3 hybridization.
The SO32- bond angle
The ideal bond angle in a tetrahedral molecule is 109.5° but in the sulfite [SO3]2- ion, the lone pair present on the central sulfur atom distorts the shape and geometry of the molecular ion. It adopts a triangular pyramidal shape and consequently, the O-S-O bond angle decreases from the ideal 109.5° to approx. 106°.
Although a S=O bond is expected to be stronger and shorter in length than an S-O bond. But, experimental results reveal that it is due to the resonance present in the sulfite ion that all three S-O bond lengths in [SO3]2- are equivalent and equal to 151 pm.
Each S-O bond present in the [SO3]2- ion is polar due to an electronegativity difference of 0.86 units between the covalently bonded sulfur (E.N = 2.58) and oxygen (E.N = 3.44) atoms. Oxygen more strongly attracts the shared S-O electron cloud as opposed to the sulfur atom. Thus each S-O bond has a specific dipole moment value (symbol µ).
The asymmetric trigonal pyramidal shape of the sulfite ion leads to an enhanced polarity effect overall. The dipole moments of individually polar S-O bonds do not get canceled equally in opposite directions. The electron cloud stays non-uniformly distributed thus sulfite [SO3]2- ion is polar with net µ > 0.
The Lewis dot structure for SO32- displays a total of 26 valence electrons i.e., 13 electron pairs.
Out of the 13 electron pairs, there are 4 bond pairs and 9 lone pairs.
Out of the 9 lone pairs, 1 lone pair of electrons is situated on the central sulfur atom.
There are 3 lone pairs on each of the singly bonded oxygen atoms while 2 lone pairs are present on the double-bonded oxygen atom in the Lewis dot structure of SO32- as shown below.
How many valence electrons are in SO32- lewis structure?
Total number of valence electrons in Sulfur = 6
Total number of valence electrons in Oxygen = 6
The [SO3]2- ion consists of 1 S-atom, 3 O-atoms, and a (-2) charge. Thus, the valence electrons in the Lewis dot structure of [SO3]2- = 1(6) + 3(6) + 2 = 26 valence electrons.
What is the difference between the Lewis dot structure of SO3 and SO32-?
The Lewis dot structure of sulfur trioxide (SO3) displays a total of 24 valence electrons while that of sulfite [SO3]2- ion displays a total of 26 valence electrons.
The 2 extra electrons in the Lewis structure of SO32- account for the negative 2 charges present on the ion. Contrarily, there are zero formal charges on all the bonded atoms in the SO3 Lewis structure.
There are 3 S=O double bonds and no lone pair on the central S atom in the SO3 Lewis structure while the SO32- Lewis structure consists of one S=O double bond, two S-O single bonds, and 1 lone pair of electrons on the central S-atom.
The ideal electron pair geometry or the actual geometry of SO32- is tetrahedral because as per the VSEPR concept it has an AX3N generic formula owing to a total of 4 electron density regions around the central S-atom, out of which one is a lone pair.
Why does SO32- ion adopt a different shape from its actual geometry?
The presence of a lone pair of electrons on the central S-atom in SO32- leads to lone pair-bond pair repulsions which distort the shape and geometry of the ion and make it adopt a different shape from its ideal electron geometry i.e., trigonal pyramidal vs. tetrahedral.
In sulfite and carbonate ions, are there a similar number of lone pairs on oxygen atoms?
Yes, both sulfite [SO3]2- and carbonate [CO3]2- ions contain three oxygen atoms bonded to the central sulfur and carbon atom respectively.
One oxygen atom is bonded via a double covalent bond to the central atom while the other two oxygen atoms are bonded to the central atom via single bonds, in both structures.
So the double-bonded oxygen has 2 lone pairs while the single-bonded oxygens contain 3 lone pairs which makes a total of 2+ 3(2) = 8 lone pairs on oxygen atoms in each of [SO3]2- and [CO3]2-.
The total number of valence electrons available for drawing sulfite [SO3]2- ion Lewis structure is 26.
The negative 2 charge present on the sulfite ion accounts for 2 extra valence electrons in its Lewis structure.
The [SO3]2- ion has a trigonal pyramidal shape and molecular geometry.
The ideal electron pair geometry of the sulfite [SO3]2- ion is tetrahedral.
The O-S-O bond angle is 106° while all the S-O bond lengths are 151 pm in [SO3]2-
It is due to the resonance present in the sulfite [SO3]2- ion that each S-O bond length is equivalent as opposed to one shorter S=O bond and two longer S-O bonds, as expected.
The central S-atom in [SO3]2- ion is sp3.
The [SO3]2- ion is polar in nature. The absence of a plane of symmetry in the molecular ion further endorses the polarity effect. Thus, the electron cloud stays non-uniformly distributed overall (net µ>0).
-1 formal charge is present on each singly bonded O-atom in the [SO3]2- Lewis structure which makes the molecular ion occupy an overall charge of -2.
In conclusion, [SO3]2- is a polyatomic ion with a double negative charge.
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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