# Sulfur trioxide (SO3) Lewis dot structure, molecular geometry or shape, electron geometry, bond angle, formal charge, hybridization

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SO3 is the chemical formula for sulfur trioxide. SO3 generally exists as a colorless liquid but when exposed to air, the liquid takes up moisture and gets converted into white fumes. SO3 is extremely valuable in the chemical manufacturing industry because it is used to make sulfuric acid.

The main goal of this article is to make you understand the chemical nature of sulfur trioxide (SO3). We have compiled for you, important information about the Lewis structure of SO3, molecular geometry or shape, electron geometry, bond angle, hybridization, and formal charge present on it. So, without any further delay, let’s start reading!

 Name of Molecule Sulfur trioxide Chemical formula SO3 Molecular geometry of SO3 Trigonal planar Electron geometry of SO3 Trigonal planar Hybridization Sp2 Nature Non-polar molecule Bond angle (O=S=O) 120° Total Valence electron in SO3 24 Overall Formal charge in SO3 Zero
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## How to draw lewis structure of SO3?

The Lewis structure of SO3 consists of a sulfur (S) atom present at the center of the molecule. It is bonded with the help of three double bonds to three atoms of oxygen (O) at the sides. There are a total of 6 lone pairs in the SO3 lewis structure.

Let’s draw the Lewis structure of SO3 using the following simple guidelines.

## Steps for drawing the Lewis dot structure of SO3

1. Count the total valence electrons in SO3

The Lewis dot structure of a molecule is a simplified representation of all the valence electrons present in the molecule. Therefore, the first step whenever we want to draw the Lewis structure of a molecule is to calculate the total valence electrons present in it.

Valence electrons present in an atomic element can be easily determined by using the Periodic Table. Having a quick look at the Periodic Table, we can readily identify that both Sulfur (S) and oxygen (O) are situated in Group VI A. So, both atoms have 6 valence electrons each.

∴ The SO3 molecule is made up of 1 S atom and 3 O atoms. Therefore, the total valence electrons available for drawing the Lewis structure of SO3 are 6 + 3(6) = 24 valence electrons.

2. Find the least electronegative atom and place it at the center

The electronegativity of an atom is defined as its ability to attract a shared pair of electrons from a covalent chemical bond. While in the Lewis structure of a molecule, the atom which is most likely to share its electrons with the other atoms is placed at the center.

So, in the Lewis structure of SO3 the least electronegative atom i.e., sulfur is placed at the center while the more electronegative oxygen atoms are placed in its surroundings, as shown in the figure below.

3. Connect outer atoms with the central atom

In this step, all the outer atoms are joined with the central atom using single bonds.

In the SO3 molecule, the 3 oxygen (O) atoms are the outer atoms while sulfur (S) is the central atom. Thus, the 3 O atoms are joined to the central S using straight lines.

Each straight line represents a single covalent bond i.e., 2 electrons. There are a total of 3 single bonds in the Lewis structure of SO3 drawn till step 3.Each bond represents 2 electrons, so the total valence electrons used till step 3 = 3(2) = 6 valence electrons.

• Total valence electrons available – electrons used till step 3 = 24 – 6 = 18 valence electrons.
• This shows we still have 18 valence electrons available to be accommodated in the Lewis structure of SO3.

4. Complete the octet of outer atoms

Oxygen (O) atoms are the outer atoms in the Lewis structure of SO3. Each O atom requires 8 valence electrons to achieve a stable octet electronic configuration.

Each O atom is bonded to the central S atom by a single bond thus each S-O bond represents an electron pair (2 electrons). This means each O atom requires 6 more electrons to complete its octet.

These 6 valence electrons are placed as 3 lone pairs around each O atom so the Lewis structure of SO3 becomes:

5. Complete the octet of the central atom and make a covalent bond if necessary

• Total valence electrons used till step 4 = 3 single bonds + 3 (electrons placed around each O atom, shown as dots) = 3(2) + 3(6) = 24 valence electrons.
• Total valence electrons available – electrons used till step 4 = 24-24 = 0 valence electrons.

All the valence electrons are already used therefore there is no lone pair on the central S atom in the Lewis structure of SO3.

But the problem that exists in the above Lewis structure is that the octet of the central S atom is still not complete. There are 3 single bonds and no lone pair around S which means it has a total of 6 valence electrons, still deficient of 2 electrons to achieve a stable octet configuration.

To solve this problem, we convert 1 of the three lone pairs present on the terminal oxygen into a covalent bond, as shown below.

Now, the central S atom has 2 single bonds + 1 double bond which means a total of 2(2) + 1 (4) = 8 valence electrons which indicates a complete octet. Therefore, the above structure is a correct Lewis representation of the SO3 molecule.

But, is this structure stable as well? Let’s check that using the formal charge concept.

6. Check the stability of Lewis’s structure with the help of the formal charge concept

The less the formal charge on the atoms of a molecule, the better the stability of its Lewis structure.

The formal charge present on two different atoms in the SO3 molecule can be determined using the formula given below.

• Formal charge = [ valence electrons – nonbonding electrons- ½ (bonding electrons)]

Now, let us count the formal charge present on SO3 using this formula and the Lewis structure shown in step 5.

For sulfur atom

• Valence electrons of sulfur = 6
• Bonding electrons = 1 double bond + 2 single bonds = 4 +2(2) = 8 electrons
• Non-bonding electrons = no lone pairs = 0 electrons
• Formal charge = 6-0-8/2 = 6-0-4 = 6-4 = +2

For double bonded oxygen atom

• Valence electrons of oxygen = 6
• Bonding electrons = 1 double bond = 4 electrons
• Non-bonding electrons = 2 lone pairs = 4 electrons
• Formal charge = 6-4-4/2 = 6-4-2 = 6-6 =0

For single bonded oxygen atoms

• Valence electrons of oxygen = 6
• Bonding electrons = 1 single bond = 2 electrons
• Non-bonding electrons = 3 lone pairs = 6 electrons
• Formal charge = 6-6-2/2 = 6-6-1 = 6-7 = -1

The above calculations show that in the Lewis structure obtained in step 5, the central sulfur (S) atom has a +2 formal charge while the two single bonded O atoms carry a -1 formal charge each.

This indicates that the Lewis structure obtained so far is not stable. Thus, we need to minimize the charges present on bonded S and O atoms in order to increase the stability of the Lewis structure. Let’s see how we can do that.

7. Minimize the formal charges on atoms by converting lone pairs to bond pairs until the most stable structure is obtained

The formal charges can be minimized by shifting a lone pair from each negatively charged O atom onto the positively charged S atom and making two covalent bonds. Refer to the figure below.

In this Lewis structure, the octet of each terminal oxygen is complete. While the central S atom has a total of 3 double bonds which means 12 valence electrons which is quite possible

This situation falls under the expanded octet rule. Sulfur can accommodate more than  8 valence electrons during chemical bonding as it has an empty 3d subshell. So, the incoming electrons are placed in the 3d subshell after completely filling the 3p orbitals.

8. Again, check the stability of the SO3 Lewis structure using the formal charge concept

For sulfur atom

• Valence electrons of sulfur =6
• Bonding electrons = 3 double bonds = 3(4) = 12 electrons
• Non-bonding electrons = no lone pair = 0 electrons
• Formal charge = 6-0-12/2 = 6-0-6 = 6-6 =0

For oxygen atoms

• Valence electrons of oxygen = 6
• Bonding electrons = 1 double bond = 4 electrons
• Non-bonding electrons = 2 lone pairs = 4 electrons
• Formal charge = 6-4-4/2 = 6-4-2 = 6-6 =0

Zero formal charges present on all the bonded atoms in the Lewis structure of SO3 are proof of its stability. The good news is that we have finally obtained the most stable Lewis structure of SO3.

In conclusion, the above structure is the best possible Lewis representation of sulfur trioxide (SO3). In reality, it is an average of the three SO3 resonance structures shown below and is known as a resonance hybrid.

The electrons present in the SO3 molecule are not stationary. Rather, they keep moving from one position to another in the molecule and this movement is called electronic delocalization.

Also check –

## What are the electron and molecular geometry of SO3?

The sulfur trioxide (SO3) molecule has an identical electron and molecular geometry i.e., trigonal planar. There are 3 electron density regions around the central S atom in the SO3 molecule and there is no lone pair present on the central atom. Thus, the molecule maintains an ideal, symmetrical shape and geometry.

### Molecular geometry of SO3

The SO3 molecule has a trigonal planar shape and molecular geometry.  The three O atoms arrange around the central S atom as 3 vertices of an equilateral triangle. The bond pair-bond pair repulsions between S=O bonds keep the O atoms at a maximum distance from one another. Therefore, the O atoms occupy terminal positions.

However, there is no lone pair present on the central S atom so lone pair-bond pair or bond pair-bond pair repulsions do not exist and the molecule maintains its symmetrical shape.

We should keep in mind that the molecular geometry or shape of a molecule gets influenced by the different bond pairs and lone pairs present on the central atom.

Contrarily, there is no such distinction between bonded and nonbonded (lone) electron pairs while considering the electron geometry of the molecule. Rather, the ideal electronic geometry of a molecule depends on the total electron density regions around the central atom.

### Electron geometry of SO3

According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, the ideal electron geometry of a molecule containing 3 regions of electron density around the central atom is trigonal planar.

In the SO3 molecule, each double bond around the central S atom is considered one region of electron density. Therefore, there are a total of 3 electron density regions around the central atom in SO3,  so its electron geometry is trigonal planar.

A simpler way of finding the electron and molecular geometry of SO3 is the AXN method.

AXN is a simple formula that is used to represent the number of bonded atoms and lone pairs present on the central atom to predict the shape or geometry of a molecule using the VSEPR concept.

AXN notation for SO3 molecule is:

• A in the AXN formula stands for the central atom present in a molecule. In the SO3 molecule, the sulfur (S) atom lies at the center so A=S.
• X represents the number of atoms bonded to the central atom. As 3 oxygen (O) atoms are bonded to the central S atom in the SO3 molecule so X=3.
• N denotes the number of lone pairs present on the central atom. As per the Lewis structure of SO3, there is no lone pair on the central S so N=0 in the SO3

Thus, the AXN generic formula becomes AX3 for the SO3 molecule.

Now let’s have a quick glance at the VSEPR Chart given below.

According to the VSEPR chart, the AX3 generic formula denotes an identical molecular and electron geometry i.e., trigonal planar, as we noted down for the SO3 molecule.

## Hybridization of SO3

The SO3 molecule has sp2 hybridization.

The electronic configuration of sulfur (S) is 1s22s22p63s23p4. During chemical bonding, one of the paired 3p electrons gets unpaired and shifts to the 3d orbital of the sulfur atom. Similarly, a 3s electron gets unpaired and shifts to another 3d orbital.

Consequently, the half-filled 3s orbital mixes with two 3p orbitals to produce three sp2 hybrid orbitals of equal energy, each containing a single electron only.

The electronic configuration of oxygen (O) is 1s2 2s2 2p4. During chemical bond formation, hybridization occurs as the 2s orbital mix with two of the three 2p orbitals of oxygen to form three sp2 hybrid orbitals. Two of the three sp2 hybrid orbitals of oxygen contain an electron pair while the third sp2 has a single electron only.

The sp2 hybrid orbitals containing electron pairs are situated as lone pairs on the oxygen atom. Contrarily,  the sp2 hybrid orbitals containing one electron only form a sigma (σ) bond with the sp2 hybrid orbitals of sulfur on each side.

The unhybridized 3p and 3d orbitals of sulfur overlap side by side with the unhybridized 2p orbitals of oxygen on each side to form pi (π) bonds. In this way, each S=O double bond has 1 sigma bond and 1 pi bond respectively, as shown in the figure below.

A shortcut for memorizing the hybridization present in a molecule is using its steric number against the table given below. The steric number of central S in the SO3 molecule is 3 as it has three regions of electron density around it. So, the table below shows that it has sp2 hybridization.

 Steric number Hybridization 2 sp 3 sp2 4 sp3 5 sp3d 6 sp3d2

## The SO3 bond angle

The SO3 molecule has an ideal electronic and molecular geometry or shape so each O=S=O bond angle in the SO3 molecule is 120°. Each S=O bond length in the molecule is 142 pm.

Also check:- How to find bond angle?

## Is SO3 polar or nonpolar?

A specific electronegativity difference of 0.86 units exists between the bonded sulfur (E.N= 2.58) and oxygen (E.N=3.44) atoms in a S=O bond.

Pauling’s electronegativity scale states that a polar bond has an electronegativity difference greater than 0.5 units between its bonded atoms.  As 0.86 > 0.5 thus each S=O bond in the SO3 molecule is polar and has a specific dipole moment (symbol μ) value.

However, it is due to the symmetrical shape of the molecule that the individual S=O dipole moments get canceled in opposite directions in the molecule overall.

The electron cloud stays uniformly distributed over the molecule. Thus, SO3 is a non-polar molecule with net μ=0.

## FAQ

### In the Lewis structure of SO3, what is the formal charge on the atom O?

SO3 molecule has three equivalent resonance structures as shown below. Each resonance structure is a way of representing the Lewis structure of SO3.

In the above structures -1 formal charge is present on the O atoms bonded to the central S atom with a single bond while 0 formal charge is present on the O atom bonded to central S via a double covalent bond.

However, in the best possible Lewis representation of SO3 shown below, the formal charge present on all the O atoms is minimized to 0 which marks the extra stability of this Lewis structure.

### How many lone pairs are present in the Lewis structure of SO3?

There is no lone pair present on the central S atom in the Lewis structure of SO3 while two lone pairs of electrons are present on each terminal O atom.

So, there are a total of 3(2) = 8 lone pairs in the Lewis structure of SO3.

### What is the difference between the Lewis structure of SO3 and SO32-?

• The sulfite [SO3]2- ion is an anion formed by the gain of two extra electrons.
• So, the total valence electrons available for drawing the Lewis structure of [SO3]2- are 26 as opposed to 24 valence electrons present in the SO3 Lewis structure.
• In the Lewis structure of [SO3 ]2- there are two single bonds and one double bond around the central S atom.
• Two of the three terminal O-atoms contain a -1 charge. This makes the ion obtain an overall charge of -2. It is unstable and thus reactive in nature.

The Lewis structure of SO3 however has 3 double bonds and no formal charge is present on any of the bonded atoms. The molecule overall is uncharged and thus stable.

### What are the similarities and differences in SO2 and SO3 Lewis structures?

Similarities:

• Both the Lewis structures contain a sulfur (S) atom at the center which is bonded to oxygen (O) atoms at the terminals.
• The total electron density regions in both the molecules are 3.

Differences:

• The Lewis structure of SO2 has a total of 18 valence electrons while the Lewis structure of SO3 displays a total of 24 valence electrons.
• There are 2 double bonds and a lone pair on the central S atom in the SO2 Lewis structure which makes it occupy a bent shape. Contrarily, there are 3 double bonds and no lone pair on central S in the Lewis structure of SO3, so it has an identical electron and molecular geometry or shape i.e., trigonal planar.

## Summary

• The total valence electrons available for drawing sulfur trioxide (SO3) Lewis structure are 24.
• The molecular geometry or shape of SO3 is trigonal planar.
• The electron geometry of SO3 is also trigonal planar.
• The molecule has an identical electron and molecular geometry as there is no lone pair present on the central S atom.
• The O atoms surrounding the central S atom occupy the three corners of an equilateral triangle in a perfectly symmetrical manner.
• SO3 molecule has sp2 hybridization.
• All the O=S=O bonds form a mutual 120° bond angle in the molecule.
• Each S=O bond length in SO3 is 142 pm.
• The polarity of individually polar S=O bonds gets canceled in opposite directions in the overall shape of the molecule thus SO3 is a non-polar molecule with net μ=0.
• There are zero formal charges present on the atoms in the resonance hybrid of SO3 which accounts for the exceptional stability of the SO3 Lewis representation.

### Vishal Goyal

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