The chemical name (IUPAC name) of SO2 is sulfur dioxide, also called sulfurous anhydrous or sulfur (IV) oxide. It is also called toxic gas as it is responsible for the smell of burnt matches.
Normally, sulfur dioxide is released during the volcanic eruption and this is how SO2 is produced naturally. It can be also synthesis artificially as a by-product of copper extraction and the burning of sulfur-containing fossil fuels.
Sulfur dioxide has a nitric acid-like pungent smell. It is found on earth with very small concentrations and also found in the atmosphere at about 1 ppm. SO2 is mainly prepared to produce sulfuric acid. Mostly the SO2 is produced by the combustion of elemental sulfur.
Properties of SO2
- It is a colorless gas with a pungent odor like a just struck match.
- It’s melting and boiling points are -72 C and -10 C respectively.
- It is soluble in water and generally forms sulfurous acid. The solubility in water is about 94 g/L.
| Name of Molecule | Sulfur dioxide |
| Chemical Notation | SO2 |
| Total valence electrons of SO2 | 18 |
| Formal charge | 0 (zero) |
| Molecular geometry of SO2 | Bent |
| Electron geometry of SO2 | Trigonal planar |
| SO2 Bond angle | 119 |
| Dipole moment of SO2 | 1.60D |
| Hybridization of SO2 | Sp2 type |
Lewis Structure of SO2
Before drawing the Lewis dot structure of any compound, we must have some basic ideas about valence electrons, the octet rule, and formal charge. Because without these basic concepts, it is quite hard to construct Lewis’s structure for any molecule. The same things will be applied for drawing SO2 Lewis structure which is given below:
What Are Valence Electrons?
Valence electrons of an atom are defined as the total electrons present on its valence shell i.e., electrons present in its outermost shell. And for a molecule, the addition of the total valence electrons of all combined atoms. In SO2, there is a total of 18 valence electrons present.
What is the Octet Rule?
The octet rule is defined as the ability of every atom in a molecule to lose or gain electrons to attain the nearest inert gas configuration. In simple words, every atom wants to have eight electrons in its outermost shell to get the best stable configuration and attain stability like a noble gas. Hydrogen and helium are two-element which do not obey the octet rule as they have a tendency to have only two electrons in their outermost shell and they will get the best stability on same.
What is the Formal Charge?
The formal charge of an atom or molecule is also called a fake charge because it is basically a charge assigned to an atom or molecule about whether it is in loss or gain or with no profit during bond formation.
We can easily calculate any molecule’s formal charge just by ignoring the lone pair of electrons and electrons shared by that atom from the total valence electrons. If there is something left, that will be assigned as a formal charge else there will not be any charge present.
In mathematical representation,
Formal Charge (FC) = [V – N – B/2] where,
V = Total number of valence electrons
N = Number of nonbonding valence electrons
B = Total number of electrons shared in bonding
Steps for drawing Lewis dot structure of SO2
Step 1: Count the total number of valence electrons
In the first step, we have to find out how many valence electrons are present in the given molecule. In the case of SO2, there is one sulfur and two oxygen atoms are present. So that first find out the valence electrons of sulfur and oxygen and then add them to get total valence electrons for SO2 molecule.
Sulfur (S) valence electrons = 6
Oxygen (O) valence electrons = 6
Here in SO2, one sulfur and two oxygen atoms are present. It means,
SO2 total Valence electrons = 6 + 6*2 = 18
Step 2: Find out the central atom & draw a simple skeleton with covalent bonds
To figure out the central atom while constructing the lewis structure is the trickiest part but not in the case of SO2 as sulfur is in lower number so that it will be the central atom surrounded by two oxygens.
Step 3: Try to fulfill octet by changing single bond to double bond if possible
With single bonds, none of the atoms have completed the octet so we have to change the single bond into a double bond and again check the octet. Now with two double bonds between sulfur and oxygen, we have used a total of 8 valence electrons.
Subtracting 8 from total valence electrons, we have still 10 more valence electrons left. The remaining valence electrons will be now placed around the atoms according to the requirements.
Starting with both oxygen atoms, we have to place two electron pairs on each oxygen and one electron pair to sulfur which finally completes the octet rule i.e., all atoms with eight electrons.
Step 4: Calculate the formal charges and check the stability
The formal charge is a very important factor for checking the stability of a molecule to determine the most stable molecular structure. As we know,
Formal charge (FC) = [No. of valence electrons – No. of bonds – 2*(No. of lone pairs)]/2
Now let’s find the formal charge for both oxygen and sulfur atoms separately,
For Oxygen atom:
Formal Charge (FC) = [6 – 2 – (2*2)]/2 = 0 where,
- No. of valence electrons = 6
- No. of bonds = 2
- Lone pairs = 2
For Sulfur atom:
Formal Charge (FC) = [6 – 2 – (2*2)]/2 = 0 where,
- No. of valence electrons = 6
- No. of bonds = 2
- Lone pairs = 2
As we can see, there is not any formal charge present on any atom so the above structure is the most stable Lewis structure of the SO2 molecule.
Also Read:
- SO3 Lewis dot structure and Molecular geometry
- CO2 Lewis dot structure, Hybridization, Shape, Polarity
SO2 Molecular Geometry and Electron Geometry
(i) Determining molecular geometry and electron geometry on the basis of electronegativity different
SO2 has a bent molecular geometry and trigonal planar electron geometry with a bond angle of 120 between both S = O bonds.
The reason for the bent geometry of SO2 is totally dependent on its constituent sulfur and oxygen atoms. Oxygen has a higher electronegativity value compared to sulfur as shared pairs of electrons slightly move towards oxygen atoms.
This uneven distribution of shared paired of electrons between oxygen and sulfur creates a partial positive charge of a sulfur atom and a partial negative charge on oxygen atoms.
The extra lone pair of electrons present on the central sulfur atom further push the S – O bond vertically as a result molecule acquires a bent molecular geometry.
(ii) SO2 molecular geometry and electron geometry based on VSEPR theory
From the given VSEPR chart we can easily find out the geometry of any molecules.
Here in this table,
- A represents the central atom,
- X represents surrounding atoms and,
- E represents the lone pairs.
Now talking about SO2 molecule, we have sulfur is the central atom, two oxygens are the surrounding atoms, and one lone pair of electrons on the central sulfur atom i.e.,
- A = S
- X = 2 and,
- E = 1
It means SO2 is an AX2E type of molecule according to the VSEPR theory and from the above table, we can easily find molecular geometry (shape) is bent and electron geometry is trigonal planar.
Here I want to clear molecular geometry and electron geometry are two different terms and they may be the same or different for a molecule that totally depends on internal electronic structure atoms in the molecule.
Electron geometry for any molecule is decided on the basis of all the corresponding electrons pair which also includes lone pairs while deciding molecular geometry is only based on the corresponding atoms.
So that if there is not any lone pair present on the central atom of any molecule, both molecular geometry and electron geometry will be the same. But if the central atom has lone pair of electrons, the molecular and electron geometry will be different.
SO2 Hybridization
SO2 has an SP2 type hybridization.
We can easily determine any of the molecule molecular orbital (i.e., hybridization) with the help of given formula which is,
No. of hybridization = [V + X – C + A]/2 where,
- V = No. of valence electrons of central atom
- X = No. of monovalent atoms around the central atom
- C = Positive charge on cation
- A = Negative charge on the anion
| Value of H | Hybridization |
| 2
3 4 5 6 7 |
sp
sp2 sp3 sp3d sp3d2 sp3d3 |
For SO2 molecule,
Hybridization (H) = [ 6+0-0+0]/2 = 3
We get H = 3, which means SO2 has an SP2 hybridization with the help of the above table.
You may be confused with why we haven’t put X =2. This is because the bond between sulfur and oxygen is not monovalent (a single bond).
At the time of hybridization, two 3P orbitals and one 3S orbital get hybridized. There is a total of three sp2 hybrid orbitals where two hybrid orbitals contain unpaired electrons, and one hybrid orbital has the lone pair of electrons. The unpaired electrons on sulfur form a sigma bond with oxygen atoms. Here both 3d and 3p orbitals remain the same and form Pi bonds within the molecule.
SO2 MO Diagram
Summary
