SO3 Lewis Structure: Drawing, Several Entities And Detailed Explanations


In this article, “so3 lewis structure” different facts about SO3 like lewis structure, formal charge, polarity and some other detailed facts on the different compounds of SO3 are discussed briefly.

SO3, known as sulphur trioxide is sp2 hybridized with a triagonal planar structure and having bond angle 1200. It is a colourless or white crystalline solid with boiling and melting point 450C and 16.90C respectively. It is a covalent compound having total three double bonds in between sulphur and oxygen are present in SO3 structure.

Main focus of this article is to explain the lewis structure, hybridization, polarity and formal charge are described below.

SO3 Lewis structure resonance

Sulphur trioxide has three resonance structures. But all the structures are equivalent because all the oxygen atoms in SO3 are equivalent.

In sulphur trioxide, one sulphur atom and three oxygen atoms are present. In SO3, sulphur is sp2 hybridized with a trigonal planar structure and bond angle is 1200. Due to this type of structure and no uneven repulsion between bond pair electrons with lone pair electrons, all the oxygen atoms are equivalent.  Not only that, all the three resonance structures contribute equally to the resonance hybrid structure.

SO3 lewis structure
SO3 Resonance Structure.
Image Credit: Wikimedia Commons.

In this above image, three resonance structures are shown and in every structure, one double bond is present between the sulphur atom with one of the three oxygen atoms. Thus, all the resonance structures are equivalent and contributes in equal amount to the hybrid resonance structure.

SO3 Lewis Structure Polar or Nonpolar

Sulphur trioxide is basically a nonpolar molecule due to its trigonal planar structure.

Polarity of any molecule depends on the electronegativity difference between two atoms in a bond. Sulphur is less electronegative than oxygen (electronegativity of sulphur and oxygen are 2.58 and 3.44 in pauling scale respectively). So, the bond between sulphur and oxygen must have polarity. But due to the orientation of three oxygen atoms around sulphur make SO3 a nonpolar molecule.

SO3, a nonpolar molecule

The resultant direction of two S=O bonds is directly opposite to the another S=O bond. So, net dipole moment is balanced by two sets of S=O bond.

Dipole moment depends on the two factors-

  • Internuclear separation between the two atoms
  • Electronic charge
  • Dipole moment = charge × internuclear separation.

It is expressed as ‘mu’(µ) and the unit is of dipole moment is debye.

SO3 Lewis structure Formal Charge

To calculate the formal charge these following formula should be kept in mind-

  • Formal charge = Total number of valance electrons – number of electrons remain as nonbonded – (number of electrons involved in bond formation/2)

Firstly, the formal charge of sulphur is calculated-

  • Formal charge of sulphur = 6 – 0 – (12/2)

                                              = 0

Similarly, formal charge of each of the oxygen is calculated-

  • Formal charge of each of the oxygen = 6- 4- (4/2)

                                                                   = 0

Valance electron of sulphur and each of the oxygen is 6 (ns2 np4) and number of nonbonded electrons for sulphur and oxygen are 0 and 4 respectively. Total numbers of electrons involving in bonding is 12 for sulphur and 4 for each of the oxygen.

SO3 Lewis structure

Lewis structure or lewis dot structure was first discovered by Gilbert.N. Lewis in the year of 1916.

This lewis dot diagram is very much helpful to determine the valance electrons, number of electrons participating in bond formation and number of nonbonded electrons.

In the lewis dot diagram of sulphur trioxide, sulphur has total 16 electrons (1s2 2s2 2p6 3s2 3p4) and among these 16 electrons, six electrons are valance electrons. These six electrons participate in covalent bond formation with oxygen. Thus, there is no nonbonded electrons in sulphur atom. All the valance electrons are used in bond formation.

Each of the oxygen also has six valance electrons (1s2 2s2 2p4) like sulphur and among these six outer shell electrons, two electrons are participating in bond formation with sulphur and rest of the four electrons are left as nonbonded electron pairs. These electrons are shown in the following image.

No nonbonded electrons are left for sulphur and four nonbonded electrons are left for each of the oxygen.

SO3H Lewis Structure

In the above structure of HSO3, two oxygen atoms are attached with sulphur by double bonds and OH group is attached with sulphur by a single bond. Thus, one valance electrons in sulphur remains as nonbonded and rest of the scenario is same like the lewis structure of sulphur trioxide.

Nonbonded electrons in oxygen are same like the SO3 because in both the structure two electron pairs remain as nonbonded pair of electrons.

SO32- Lewis structure

The lewis structure of SO32- is almost similar to the neutral SO3. The basic difference with SO3 is that in SO32-, sulphur is attached by one double bond with one oxygen atom and another two oxygen atoms are attached with single bonds and bearing two negative charges.

Thus, two pair of electrons remain as nonbonded for the oxygen attached by single bond with sulphur and one pair of electrons left as nonbonded for the oxygen attached by double bond with sulphur. As usual no electrons are left as nonbonded for sulphur atom because all the valance electrons are used up in covalent bond formation with oxygen.

SO31- Lewis structure

In this lewis structure, sulphur is attached by two oxygen atoms by double bonds and rest of the oxygen is attached by one single bond with oxygen. Thus, one lone electron remains as nonbonded for sulphur atom and for single bonded oxygen, six electrons remain as nonbonded. For this reason, the total molecule bears one negative charge as a whole. The doubly bonded oxygen has four electrons as nonbonded and formal charge for this oxygen atoms are zero.

Is SO3 Ionic Or Covalent?

Sulphur trioxide or SO3 is definitely a covalent compound. Covalent bonds are generally formed between two nonmetal atoms and electronegativity difference between the atoms participating in covalent bond formation is not so high.

Similarly, in SO3, sulphur and oxygen are two nonmetal atoms and electronegativity difference between sulphur and oxygen is not very high.

Due to small difference in electronegativity (electronegativity of sulphur and oxygen are 2.58 and 3.44 in pauling scale respectively) electrons cannot be shared equally between sulphur and oxygen. The bonded electrons are attracted more by the oxygen due to greater electronegativity with respect to sulphur. Thus, the nature of the covalent bond between sulphur and oxygen is polar but the molecule itself is a nonpolar due to completely balancing of two opposite dipole moment.

Frequently Asked Questions (FAQ)

Is SO3 reactive with water?

Answer: Yes, sulphur trioxide reacts with water and form sulphuric acid (H2SO4). This reaction is known as hydrolysis reaction.

SO3 + H2O = H2SO4

Is SO3 acid or base?

Answer: SO3 is a lewis acid because sulphur is electron deficient can accept electron pairs from electron rich atoms.

Aditi Roy

Hi..I am Aditi Roy, I have completed my graduation from University of Calcutta and am pursuing M.Sc in Chemistry from Techno India University. My area of specialization is on Inorganic Chemistry.

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