Simple Procedure for writing Lewis Structures – Lewis Structures for sulfur trioxide (SO3)

A simple procedure for writing Lewis structures is given in a previous article entitled “Lewis Structures and the Octet Rule”. Relevant worked examples were given in the following articles: Examples #1, #2, #3 and #4.

A few more examples for writing Lewis structures following the above procedure are given bellow:

Let us consider the case of sulfur trioxide SO₃ :

Step 1: The central atom will be the S atom since it is the less electronegative. Connect the atoms with single bonds:

Step 2:  Calculate the # of electrons in π bonds (multiple bonds) using formula (1) in the article entitled “Lewis Structures and the Octet Rule”. 

Where n in this case is 4 since SO₃ consists of four atoms.

Where V = (6 + 6 + 6 + 6) = 24  

Therefore, P = 6n + 2 – V = 6 * 4 + 2 – 24 = 2      \  there are 2 π electrons in SO₃  \

1 double bond must be added to the structure of Step 1.

Step 3 & 4: One double bond must therefore be placed through the 3 S-O bonds. Therefore, the Lewis structures for SO₃ are as follows:

Figure 1: Lewis structures for SO₃. In this case the sulfur atom, using valency shell n =3 is not restricted to an octet, and additional double bonds may be generated from the oxygen lone pairs to equalize the charge. Resonance structure 4 is the most probable one since the there is no charge separation.