Sulfur Trioxide (SO3) Lewis Structure, Hybridization | Drawing Steps

Sulfur trioxide molecule contains one sulfur atom and three oxygen atoms. We will construct the lewis structure of SO3 molecule by following VSEPR theory rules and considering stability of intermediate structures. Finally, after obtaining the lewis structure of SO3, we can determine the hybridization of atoms.

Sulfur trioxide | SO3

Sulfur trioxide is a oxide of sulfur and colourless inorganic gas. Also it is a toxic gas. Sulfur trioxide gas is produced due to oxidation of sulfur dioxide gas in air.

Lewis structure of SO3 molecule

There are three double bonds around sulfur atom with oxygen atoms in SO molecule. Each oxygen atom has two lone pairs in SO3 lewis structure. But, there is no lone pair on sulfur atom in SO3 lewis structure as lewis structure of SO2.

SO3 lewis structure - (sulfur trioxide)

Hybridization of SO3 molecule

All atoms have sp2 hybridization. Each oxygen atom has one sigma bond and two lone pairs. Therefore, oxygen atoms' hybridization should be sp2. For sulfur atom, there are three sigma bonds and no lone pair and it's hybridization is also sp2.

hybridization of SO3 (sulfur trioxide)
Simple method to determine the hybridization of atoms

Apply VSEPR theory - Steps of drawing lewis structure of SO3 molecule

Following steps are the main guidelines we have to use for drawing the lewis structure of SO3. Each step is explained in detail in next sections. If you are a beginner to lewis structure drawing, follow these sections slowly and properly to understand it completely. Look the figures to understand each step.

  1. Find total number of electrons of the valance shells of sulfur and oxygen atoms
  2. Total electrons pairs
  3. Center atom selection
  4. Put lone pairs on atoms
  5. Check the stability and minimize charges on atoms by converting lone pairs to bonds until most stable structure is obtained.

steps of drawing SO3 lewis structure - VSEPR method

Total number of electrons of the valance shells of ethene

Both sulfur and oxygen belongs to the group VIA elements' series. Therefore, they have six electrons in their valence shell. To find number of valence electron, these valence electrons of each element should be multiplied with their respective number of atoms in the molecule. Below, That step are done.

  • Total valence electrons given by three oxygen atoms = 6 * 3 = 18

  • Total valence electrons given by sulfur atom = 6 * 1 = 6

There are no charges in SO3 molecule. Therefore, no addition or reduction of valence electrons due to charges.

  • Total valence electrons = 18 + 6 = 24

Total valence electrons pairs

Total valance electrons pairs = σ bonds + π bonds + lone pairs at valence shells

Total electron pairs are determined by dividing the number total valence electrons by two. For, SO3 molecule, Total number of pairs of electrons = 24/2 = 12.

Center atom and sketch of ethene molecule

There are several requirements to be the center atom in a molecule. We have to think whether center atom is sulfur or oxygen. Having a high valence is a main requirement to be a center atom. For SO3 molecule, sulfur has the highest valence than and oxygen. Therfore, sulfur is the center atom in SO3. Structure of atoms of SO3 is figured below.

SO3 sketch

Mark lone pairs on atoms

After drawing the sketch, we should start to mark lone pairs on atoms. In the drawn sketch, there are three bonds between atoms.

  • There are already three S-O bonds in the above sketch. Now, there are only nine (12-3 = 9) valence electrons pairs are remaining to draw (as mark lone pairs) the rest of the structure.
  • First, mark remaining valence electrons pair as lone pairs on oxygen atoms (outside atoms). Nine valence electron pairs are marked on three oxygen atoms. Now, all remaining nine valence electron pairs are marked.
  • Therefore, there is no lone pair to mark on sulfur atom (That is not a problem at this stage).
mark electron pairs on SO3 sketch

Charges on atoms

Charges on atoms are important to find the most stable structure. Therefore, we need to find the most stable structure to obtain lewis structure. Therefore, we should try to find charges if there are.

After, marking electron pairs on atoms, we should mark charges of each atom. Each oxygen atoms will get a -1 charge and sulfur atom get a +3 charge. Because SO2 is a neutral molecule, overall charge of the molecule should be zero. The overall charge of the molecule is, (-1) * 3 + (+3) = 0.

mark charges on atoms in sulfur trioxide

Stability of structure and minimize charges on atoms by converting lone pairs to bonds

When there are positive and negative charges on lot of atoms or higher charges (like +2, +3, -2, -3) on atoms in an ion or molecule, that structure is not stable one. Therefore, We should try to reduce charges on atoms if it is a possible. In the above structure, there are charges on oxygen atoms and sulfur atom. Now, we are going to reduce charges on these atoms as below.

  • Step 1: Now, we should try to minimize charges by converting a lone pair or pairs to a bond. So convert a lone pair on a oxygen atom to make a new S-O bond with sulfur atom as the following figure.
  • Now there is a double bond between one oxygen atom and sulfur atom. You can see, charges are reduced now in the new structure.
  • Step 2: Again, we can convert a lone pair on another oxygen atom to make a S-O bond. Now, another double bond is formed. In the new structure, there are two double bonds.
  • Step 3: Because, there are still charges on atoms, we can try to convert a lone pair to a bond. So, convert a lone pair on other oxygen atom to make a bond with sulfur atom. With that, there are no charges on sulfur and oxygen atoms and that structure is stable than other previous structures.
  • Therfore, that structure should be the lewis structure of SO2
reduce charges of atoms in SO3


Lewis Structures of Molecules