SO3 Lewis Structure in 6 Steps (With Images)

So you have seen the above image by now, right?

Let me explain the above image in short.

SO3 lewis structure has a Sulfur atom (S) at the center which is surrounded by three Oxygen atoms (O). There are 3 double bonds between the Sulfur atom (S) and each Oxygen atom (O). There are 2 lone pairs on all three Oxygen atoms (O).

If you haven’t understood anything from the above image of SO3 (sulfur trioxide) lewis structure, then just stick with me and you will get the detailed step by step explanation on drawing a lewis structure of SO3.

So let’s move to the steps of drawing the lewis structure of SO3.

Steps of drawing SO3 lewis structure

Step 1: Find the total valence electrons in SO3 molecule

In order to find the total valence electrons in SO3 (sulfur trioxide) molecule, first of all you should know the valence electrons present in sulfur atom as well as oxygen atom.
(Valence electrons are the electrons that are present in the outermost orbit of any atom.)

Here, I’ll tell you how you can easily find the valence electrons of sulfur as well as oxygen using a periodic table.

Total valence electrons in SO3 molecule

→ Valence electrons given by sulfur atom:

Sulfur is a group 16 element on the periodic table. ^[1] Hence the valence electrons present in sulfur is 6.

You can see the 6 valence electrons present in the sulfur atom as shown in the above image.

→ Valence electrons given by oxygen atom:

Oxygen is group 16 element on the periodic table. ^[2] Hence the valence electrons present in oxygen is 6.

You can see the 6 valence electrons present in the oxygen atom as shown in the above image.

Hence, 

Total valence electrons in SO3 molecule = valence electrons given by 1 sulfur atom + valence electrons given by 3 oxygen atoms = 6 + 6(3) = 24.

Step 2: Select the central atom

For selecting the center atom, you have to remember that the atom which is less electronegative remains at the center.

Now here the given molecule is SO3 (sulfur trioxide) and it contains sulfur atom (S) and oxygen atoms (O).

You can see the electronegativity values of sulfur atom (S) and oxygen atom (O) in the above periodic table.

If we compare the electronegativity values of sulfur (S) and oxygen (O) then the sulfur atom is less electronegative.

So here the sulfur atom (S) is the center atom and the oxygen atoms (O) are the outside atoms.

Step 3: Connect each atoms by putting an electron pair between them

Now in the SO3 molecule, you have to put the electron pairs between the sulfur atom (S) and oxygen atoms (O).

This indicates that the sulfur (S) and oxygen (O) are chemically bonded with each other in a SO3 molecule.

Step 4: Make the outer atoms stable

Now in this step, you have to check the stability of the outer atoms.

Here in the sketch of SO3 molecule, you can see that the outer atoms are oxygen atoms.

These outer oxygen atoms are forming an octet and hence they are stable. 

Also, in step 1 we have calculated the total number of valence electrons present in the SO3 molecule.

The SO3 molecule has a total 24 valence electrons and all these valence electrons are used in the above sketch of SO3. 

Hence there are no remaining electron pairs to be kept on the central atom. 

So now let’s proceed to the next step.

Step 5: Check the octet on the central atom. If it does not have octet, then shift the lone pair to form a double bond or triple bond.

In this step, you have to check whether the central sulfur atom (S) is stable or not.

In order to check the stability of the central sulfur (S) atom, we have to check whether it is forming an octet or not.

Unfortunately, the sulfur atom is not forming an octet here. Sulfur has only 6 electrons and it is unstable.

Now to make this sulfur atom stable, you have to shift the electron pair from the outer oxygen atom so that the sulfur atom can have 8 electrons (i.e octet).

After shifting this electron pair, the central sulfur atom will get 2 more electrons and thus its total electrons will become 8.

You can see from the above picture that the sulfur atom is forming an octet as it has 8 electrons.

Now let’s proceed to the final step to check whether the lewis structure of SO3 is stable or not.

Step 6: Check the stability of lewis structure

Now you have come to the final step in which you have to check the stability of lewis structure of SO3.

The stability of lewis structure can be checked by using a concept of formal charge.

In short, now you have to find the formal charge on sulfur (S) atom as well as oxygen (O) atoms present in the SO3 molecule.

For calculating the formal charge, you have to use the following formula;

Formal charge = Valence electrons – (Bonding electrons)/2 – Nonbonding electrons

You can see the number of bonding electrons and nonbonding electrons for each atom of SO3 molecule in the image given below.

For Sulfur (S) atom:
Valence electrons = 6 (because sulfur is in group 16)
Bonding electrons = 8
Nonbonding electrons = 0

For double bonded Oxygen (O) atom:
Valence electrons = 6 (because oxygen is in group 16)
Bonding electrons = 4
Nonbonding electrons = 4

For single bonded Oxygen (O) atom:
Valence electrons = 6 (because oxygen is in group 16)
Bonding electrons = 2
Nonbonding electrons = 6

Formal charge	=	Valence electrons	–	(Bonding electrons)/2	–	Nonbonding electrons
S	=	6	–	8/2	–	0	=	+2
O (double bonded)	=	6	–	4/2	–	4	=	0
O (single bonded, 1st)	=	6	–	2/2	–	6	=	-1
O (single bonded, 2nd)	=	6	–	2/2	–	6	=	-1

From the above calculations of formal charge, you can see that the sulfur (S) atom has +2 charge and both the single bonded oxygen (O) atoms have -1 charges.

Because of this reason, the above obtained lewis structure of SO3 is not stable. 

So we have to minimize these charges by shifting the electron pairs towards the sulfur atom.

After shifting the electron pairs from oxygen atom to sulfur atom, the lewis structure of SO3 becomes more stable.

In the above lewis dot structure of SO3, you can also represent each bonding electron pair (:) as a single bond (|). By doing so, you will get the following lewis structure of SO3.

I hope you have completely understood all the above steps.

For more practice and better understanding, you can try other lewis structures listed below.

Try (or at least See) these lewis structures for better understanding:

C2H4 lewis structure	SF4 lewis structure
H2S lewis structure	OF2 lewis structure
NF3 lewis structure	CCl4 lewis structure