Sulfur trioxide, SO3, is one of the two most important sulfur oxides in the atmosphere. There is so much to know about this molecule. In this article, I’ll discuss the molecular geometry of SO3, its hybridization, resonance, and everything about its structure.
Understanding the molecular geometry is a lot more than it seems. It helps to understand how and why the molecule behaves the way it does in chemical reactions. It is also crucial to predicting its reactivity, polarity, color, and attraction.
SO3 is an important molecule in the chemistry and chemical manufacturing industries. It is a precursor to sulphuric acid and is used in the manufacture of acids, fertilizers, lead-acid batteries, and in the purification of petroleum. However, this is not the focus of this article.
The Lewis structure and Valence Shell Electron Pair Repulsion (VSEPR) theory explains the molecular geometry of SO3.
Properties of SO3
- SO3 is the chemical formula for sulfur trioxide. It has a molar mass of 80.066 g/mol
- It is an oxide of sulfur that usually exists as a colorless and odorless liquid. But when the liquid absorbs moisture, it becomes the toxic SO3 fumes
- Sulfur trioxide also exists as gaseous monomers, crystalline trimers, and solid polymers
- Just below temperature, SO3 is a white crystalline solid that fumes in the air
- In its liquid state, SO3 has a density of 1.92 g/cm3
- It has a melting point of 62.4°F (16.9°C) and a boiling point of 113°F (45°C)
- SO3 is soluble in water and reacts with water to form sulfuric acid
What is the molecular geometry of SO3?
SO3 has a trigonal planar molecular geometry. The sulfur and oxygen atoms are stretched far apart from each other. The three oxygen atoms are arranged around the central sulfur atom as vertices of a triangle.
According to the VSEPR theory, electron pairs repel themselves and arrange themselves as far as possible from each other. Because of this, there are bond pair-bond pair repulsions between the S=O bonds.
This makes the oxygen atoms stay far apart and give the molecule its trigonal planar molecular geometry.
What is the electron geometry of SO3?
SO3 has the same electron geometry as its molecular geometry. The reason for this is the absence of any lone pair electrons on the overall molecule. The presence of lone pair electrons has a way of distorting the structure of molecules and giving them another electron pair geometry.
Since there are no lone pair electrons on the sulfur atom, there are no lone pair-bond pair repulsions. As a result, the molecule retains its symmetrical shape.
Furthermore, this can be verified using the VSEPR theory of chemical bonding. In the SO3 molecule, there are three S=O bonds. Each double bond around the central sulfur atom is a region of electron density.
Since there are three double bonds around the sulfur atom, there are three regions of electron density. According to the VSEPR theory of chemical bonding, molecules with three regions of electron density around the central atom have a trigonal planar electron geometry.
Additionally, using the VSEPR chart, the AX3 generic formula means a molecule has identical molecular and electron geometry.
What is the hybridization of SO3?
SO3 has an sp2 hybridization and a hybridization or steric number of 3.
The hybridization can be determined by finding the sum of total sigma bonds and lone pairs in the molecule. From the Lewis stricture of SO3, there are three double covalent bonds. Each double covalent bond contains one sigma bond and one pi bond.
That is, there is a total of three sigma bonds and no lone pair (3 + 0 = 3). The sp2 hybridization shows an overlap of one s orbital and two p orbitals in the same shell. The three orbitals mix to produce three new hybrid orbitals of similar energy levels.
Furthermore, the hybridization of SO3 supports its trigonal planar geometry. Each O=S=O bond has a bond angle of 120° and each S=O bond has a bond length of 142 pm.
Is SO3 a polar or non-polar molecule?
Overall, SO3 is a non-polar molecule. Each S=O bonds are however polar. Between these two bonded atoms is an electronegativity difference of 0.86. Pauling’s electronegativity scale says that the bond between two atoms is polar if the electronegativity difference is greater than 0.5.
Each S=O bond has an electronegativity difference greater than 0.5. Therefore, they are polar bonds and have dipole moments. However, the molecule has a symmetrical shape which makes the dipole moments cancel out each other.
As a result, the overall dipole moment on the molecule is zero and the molecule is non-polar.
Does SO3 show resonance?
SO3 can have seven resonance structures but only three have proven true and valid.
The first structure has a formal charge of 0. In the second structure, sulfur has a formal charge of +1, and one of the oxygen atoms has a formal charge of -1. In the third structure, sulfur has a formal charge of +2 and each oxygen atom has a formal charge of -1.
Resonance occurs when there is a possibility of drawing multiple Lewis structures for one molecule. The difference between the structures is the placement of electrons.
There is more than one way of distributing the electron pairs around the sulfur and oxygen atoms to get a more stable overall structure. This means that every stricture must satisfy the octet rule for sulfur and all oxygen atoms.
What are the reactions SO3 undergoes?
Hydration
SO3 is the acid anhydride of H2SO4. It reacts with water to form sulfuric acid in an exothermic reaction. This occurs in a series of steps that involve hydration and proton transfer.
SO3 + H2O ———> H2SO4
Hydrofluorination
Just like water hydrates SO3 to form an acid, hydrogen fluoride reacts with it to form fluorosulfuric acid.
SO3 + HF ———> FSO3H
Reaction with bases
SO3 reacts with bases to form sulfates and release a lot of heat. For instance, it reacts with sodium hydroxide to form sodium hydrogen sulfate initially. In excess sodium hydroxide, it forms sodium sulfate.
SO3 + NaOH ———> Na2SO4
Reaction with ammonia
SO3 reacts with ammonia gas to form ammonium sulfate and release heat.
SO3 + NH3 ———> (NH4)2SO4
Decomposition
At high temperatures between 1,250.33°F (676.8°C) to 1,790.33°F (976.8°C), SO3 will decompose into SO2 and O2. This is a key reaction in sulfur-based thermochemical water-splitting cycles during the production of hydrogen.
SO3 ———> SO2 + O2
FAQs
Is SO3 more polar than SO2?
No, it is not. SO3 is a non-polar molecule. SO2, on the other hand, is a bent molecule and is polar. This is why it has a higher boiling point than SO3.
Is SO3 a coordinate covalent bond?
Coordinate covalent bonds are formed with an electron pair. SO3 has no coordinate covalent bond because no bond was formed by contributing an electron pair.
Does SO3 obey the octet rule?
SO3 does not obey the octet rule because it forms three double bonds with the oxygen atoms. it however forms a stable compound with a total of 24 electrons in the outermost shells of the sulfur and oxygen atoms.
This can happen because sulfur, the central atom, belongs to a group of elements that tend to expand their octet to accommodate more than the standard 8 electrons.
Conclusion
The three oxygen atoms in the SO3 molecule lie away from the central sulfur atom in a triangular style, which gives it its trigonal planar molecular geometry. It has the ideal 120° bond angle expected for trigonal planar molecules, based on the VSEPR theory.
The molecular geometry of any molecule is crucial to understanding its reactivity and role in chemical reactions. It also explains its stability, its defiance of the octet rule notwithstanding.
You can also learn about the molecular geometry of a similar sulfur oxide, SO2.
Thanks for reading.