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 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.
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.
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.
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.
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.
There are no charges in SO3 molecule. Therefore, no addition or reduction of valence electrons due to charges.
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.
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.
After drawing the sketch, we should start to mark lone pairs on atoms. In the drawn sketch, there are three bonds between 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.
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.