SO2 (sulfur dioxide) has one sulfur atom and two oxygen atoms.
In the lewis structure of SO2, there are two double bonds around the sulfur atom, with two oxygen atoms attached to it. On each oxygen atom, there are two lone pairs, and on the sulfur atom, there is one lone pair.
Here’s how you can draw the SO2 lewis structure step by step.
Step #1: draw sketch
Step #2: mark lone pairs
Step #3: mark charges
Step #4: minimize charges
Step #5: minimize charges again (if there are)
Let’s break down each step in detail.
#1 Draw Sketch
- First, determine the total number of valence electrons
In the periodic table, both sulfur and oxygen lie in group 16.
Hence, both sulfur and oxygen have six valence electrons.
Since SO2 has one sulfur atom and two oxygen atoms, so…
Valence electrons of one sulfur atom = 6 × 1 = 6
Valence electrons of two oxygen atoms = 6 × 2 = 12
And the total valence electrons = 6 + 12 = 18
Learn how to find: Sulfur Valence Electrons and Oxygen Valence Electrons
- Second, find the total electron pairs
We have a total of 18 valence electrons. And when we divide this value by two, we get the value of total electron pairs.
Total electron pairs = total valence electrons ÷ 2
So the total electron pairs = 18 ÷ 2 = 9
- Third, determine the central atom
We have to place the least electronegative atom at the center.
Since sulfur is less electronegative than oxygen, assume that the central atom is sulfur.
Therefore, place sulfur in the center and oxygens on either side.
- And finally, draw the rough sketch
#2 Mark Lone Pairs
Here, we have a total of 9 electron pairs. And two S — O bonds are already marked. So we have to only mark the remaining seven electron pairs as lone pairs on the sketch.
Also remember that sulfur is a period 3 element, so it can keep more than 8 electrons in its last shell. And oxygen is a period 2 element, so it can not keep more than 8 electrons in its last shell.
Always start to mark the lone pairs from outside atoms. Here, the outside atoms are oxygens.
So for each oxygen, there are three lone pairs, and for sulfur, there is one lone pair.
Mark the lone pairs on the sketch as follows:
#3 Mark Charges
Use the following formula to calculate the formal charges on atoms:
Formal charge = valence electrons – nonbonding electrons – ½ bonding electrons
For sulfur atom, formal charge = 6 – 2 – ½ (4) = +2
For each oxygen atom, formal charge = 6 – 6 – ½ (2) = -1
Here, both sulfur and oxygen atoms have charges, so mark them on the sketch as follows:
The above structure is not a stable lewis structure because both sulfur and oxygen atoms have charges. Therefore, reduce the charges (as below) by converting lone pairs to bonds.
#4 Minimize Charges
Convert a lone pair of the oxygen atom to make a new S — O bond with the sulfur atom as follows:
#5 Minimize Charges Again
Since there are charges on sulfur and oxygen atoms, again convert a lone pair of the oxygen atom to make a new S — O bond with the sulfur atom as follows:
In the above structure, you can see that the central atom (sulfur) forms an octet. And the outside atoms (oxygens) also form an octet. Hence, the octet rule is satisfied.
Also, the above structure is more stable than the previous structures. Therefore, this structure is the stable lewis structure of SO2.
Next: NO2 Lewis Structure