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

Lewis structure of sulfur trioxide SO3

Lewis Electron Dot Structure of sulfur trioxide SO3

A simple procedure for writing Lewis Structures was given in a previous article entitled Lewis Structures and the Octet Rule”. Several worked examples relevant to this procedure were given in previous posts please see the Sitemap - Table of Contents (Lewis Electron Dot Structures).

Consider the case of sulfur trioxide, SO3. SO3 is used for the industrial production of sulfonates - organic compounds with a sulfur carbon bonds - when it reacts with organic compounds. A well known and mass produced sulfonate is sodium dodecylbenzenesulfonate (also commonly referred to as linear alkylbenzene sulfonate or LAS) which is a series of organic compounds with the formula C12H25C6H4SO3Na. It is a colourless salt with useful properties as a surfactant. It is usually produced as a mixture of related sulfonates. LAS is a major component of laundry detergent.

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

Fig I.1: Atoms connected with single bonds in SO3


Step 2: Calculate the # of electrons in π bonds (multiple bonds) using formula (1) :

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      So there are 2 π electrons in SO3 and therefore 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 of SO3 are as follows:

Figure 2: 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.


The above simple procedure for drawing Lewis electron dot structures is also shown in the following video:





Relevant Posts

Lewis structure of SO2 – Simple Procedure for drawing Dot structures

Simple Procedure for writing Lewis Structures- CO2, NCO-

Lewis Dot Structure of the sulfite ion SO3-2

Lewis Structures|Octet Rule: A Simple Method to write Lewis Structures



  1. G.N. Lewis, J.A.C.S, 38, 762-785, (1916)
  2.  E. C. McGoran, J. Chem. Educ., 68, 19-23 (1991)
  3. A.B.P. Lever, J. Chem. Educ., 49, 819-821, (1972)


Key Terms

resonance structures of sulfur trioxide SO3, Lewis electron structures of SO3, chemical formula of SO3, simple method for drawing Lewis structures of SO3,


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