Lewis Structure of SO3 (With 5 Simple Steps to Draw!)

Ready to learn how to draw the lewis structure of SO3?

Awesome!

Here, I have explained 6 simple steps to draw the lewis dot structure of SO3 (along with images).

So, if you are ready to go with these 6 simple steps, then let’s dive right into it!

Lewis structure of SO3 (or Sulfur trioxide) contains three double bonds between the Sulfur (S) atom and each Oxygen (O) atom. The Sulfur atom (S) is at the center and it is surrounded by 3 Oxygen atoms (O). The Sulfur atom does not have a lone pair while all the three Oxygen atoms have 2 lone pairs.

Let’s draw and understand this lewis dot structure step by step.

(Note: Take a pen and paper with you and try to draw this lewis structure along with me. I am sure you will definitely learn how to draw lewis structure of SO3).

6 Steps to Draw the Lewis Structure of SO3

Step #1: Calculate the total number of valence electrons

Here, the given molecule is SO3 (sulfur trioxide). In order to draw the lewis structure of SO3, first of all you have to find the total number of valence electrons present in the SO3 molecule.
(Valence electrons are the number of electrons present in the outermost shell of an atom).

So, let’s calculate this first.

Calculation of valence electrons in SO3

• For Sulfur:

Sulfur is a group 16 element on the periodic table. ^[1]

Hence, the valence electrons present in sulfur is 6 (see below image).

• For Oxygen:

Oxygen is also a group 16 element on the periodic table. ^[2]

Hence, the valence electron present in oxygen is also 6 (see below image).

Hence in a SO3 molecule, 

Valence electrons given by Sulfur (S) atom = 6
Valence electrons given by each Oxygen (O) atom = 6
So, total number of Valence electrons in SO2 molecule = 6 + 6(3) = 24

Step #2: Select the center atom

While selecting the atom, always put the least electronegative atom at the center. 

(Remember: Fluorine is the most electronegative element on the periodic table and the electronegativity decreases as we move right to left in the periodic table as well as top to bottom in the periodic table). ^[3]

Here in the SO3 molecule, if we compare the sulfur atom (S) and oxygen atom (O), then the sulfur is less electronegative than oxygen.

So, sulfur should be placed in the center and the remaining 3 oxygen atoms will surround it.

Step #3: Put two electrons between the atoms to represent a chemical bond

Now in the above sketch of SO3 molecule, put the two electrons (i.e electron pair) between each sulfur atom and oxygen atom to represent a chemical bond between them.

These pairs of electrons present between the Sulfur (S) and Oxygen (O) atoms form a chemical bond, which bonds the sulfur and oxygen atoms with each other in a SO3 molecule.

Step #4: Complete the octet (or duplet) on outside atoms. If the valence electrons are left, then put the valence electrons pair on the central atom

Don’t worry, I’ll explain!

In the Lewis structure of SO3, the outer atoms are oxygen atoms.

So now, you have to complete the octet on these oxygen atoms (because oxygen requires 8 electrons to have a complete outer shell).

Now, you can see in the above image that all the oxygen atoms form an octet.

Also, all the 24 valence electrons of SO3 molecule (as calculated in step #1) are used in the above structure. So there are no remaining electron pairs.

Hence there is no change in the above sketch of SO3.

Let’s move to the next step.

Step #5: Check whether the central atom has octet or not. If it does not have an octet, then move the electron pair from the outer atom to form a double bond or triple bond

In this step, we have to check whether the central atom (i.e sulfur) has an octet or not. 

In simple words, we have to check whether the central Sulfur (S) atom has 8 electrons or not.

As you can see from the above image, the central atom (i.e sulfur) has only 6 electrons. So it does not fulfill the octet rule.

Now, in order to fulfill the octet of sulfur atom, we have to move the electron pair from the outer atom (i.e oxygen atom) to form a double bond.

Now you can see from the above image that the central atom (i.e sulfur), is having 8 electrons. So it fulfills the octet rule.

Step #6: Final step – Check the stability of lewis structure by calculating the formal charge on each atom

Now, you have come to the final step and here you have to check the formal charge on sulfur atom (S) as well as each oxygen atom (O).

For that, you need to remember the formula of formal charge;

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

• For Sulfur:
  Valence electrons = 6 (as it is in group 16)
  Nonbonding electrons = 0
  Bonding electrons = 8
• For double bonded Oxygen:
  Valence electron = 6 (as it is in group 16)
  Nonbonding electrons = 4
  Bonding electrons = 4
• For single bonded Oxygen:
  Valence electron = 6 (as it is in group 16)
  Nonbonding electrons = 6
  Bonding electrons = 2

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

So you can see above that the formal charges on sulfur is +2 and the formal charge on both the single bonded oxygen atoms is -1.

This indicates that the above lewis structure of SO3 is not stable and so we have to minimize the charges to get a more stable lewis structure.

This can be done by shifting the lone pair from negatively charged oxygen atoms to the positively charged sulfur atom to form a bond.

Now, in the above structure, you can see that the charges are minimized and the above lewis structure of SO3 is the final stable structure.

Each electron pair (:) in the lewis dot structure of SO3 represents the single bond ( | ). So the above lewis dot structure of SO3 can also be represented as shown below.

Related lewis structures for your practice:
Lewis structure of C2H4
Lewis structure of SF4
Lewis structure of H2S
Lewis structure of OF2
Lewis structure of NF3

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