Structure, Molecular Geometry, and Hybridization of SO3.

Sulfur trioxide is a chemical compound having the formula SO3. As it combines with the water in the air to form sulfuric acid, this chemical is of tremendous relevance and is extensively studied.

When this gaseous sulfuric acid combines with rain and falls to Earth, it is referred to as acid rain. Sulfur trioxide is typically colourless and greasy, but extremely corrosive.

Also known as battery acid, it is utilised in the production of acids, fertilisers, lead-acid batteries, metal pickling, and petroleum purification, among other applications.

The Lewis structure, which specifies the number of valence electrons present in an atom, is also referred to as an electron dot structure.

In addition, they describe how these valence electrons contribute to the development of molecules’ bonds.

In addition, the Lewis structure aids in determining the quantity and type (single, double, or triple) of the bonds represented by lines.

This structure is essential for anyone beginning their study of atomic chemistry.

To comprehend these graphs, it is necessary to comprehend how the number of valence electrons was determined. Sulfur has an atomic number of 16, making its electrical configuration 1s2 2s2 2p6 3s2 3p4.

As the p shell must accept 6 electrons, two more electrons are required to complete the 3p shell. Conversely, oxygen has an atomic number of eight, making its electrical arrangement 1s2 2s2 2p4.

Again, two additional electrons are required to maintain the 2p shell. Now, the Lewis structure must be drawn so that all 3p shells and all 2p shells are filled.

Valence Electrons

The electrons in an atom’s outermost shell that readily participate in bond formation are known as valence electrons.

The weaker grip of the nucleus on these electrons, along with their small quantity, enables them to participate in bond formation.

In the instance of sulphur trioxide, each sulphur atom contributes six valence electrons and three oxygen atoms combine to form a molecule.

Octet Rule

The maximum number of electrons that can occupy a valence shell, according to the octet rule, is eight.

Only when there are fewer than eight electrons does an atom undergo bond formation by absorbing or donating valence electrons in order to establish a stable state similar to that of noble gases.

In both sulphur and oxygen, two valence electrons are in short supply.

How does Sulfur break the octet rule?

Sulfur, which belongs to period 3 of the periodic table, is the exception.

Because sulphur can expand its octet and accommodate up to 12 electrons, it does not adhere to the octet rule.

Sulfur can form bonds because it has access to energetic 3d-subshells.

Due to the modest energy difference between the 3p and 3d shells, an unpaired electron can easily migrate from the 3p shell to the 3d shell with the aid of a little excitation.

In the event that all valence electrons are located in the 3s and 3p shells, sulphur can form up to two covalent bonds and two lone pairs.

When one valence electron reaches the 3d shell, however, sulphur can form four covalent bonds and one lone pair.

It is crucial to comprehend that sulfur’s octet can expand to accommodate a maximum of ten to twelve electrons.

so3 electrical configuration

When an additional valence electron enters the 3d shell, the total number of unpaired electrons in sulphur increases to six.

Sulfur may now make six covalent bonds, allowing twelve electrons to surround its valence shell.

The development of additional covalent bonds results in the release of additional energy, making the final structure significantly more stable.

SO3’s Lewis structure

Sulfur trioxide is a tetraatomic chemical compound in which each sulphur and oxygen molecule binds with three valence electrons.

The diagram depicts dots of valence electrons surrounding the symbols for both sulphur and oxygen atoms, along with lines that forecast bond formation.

The following formula represents the Lewis structure of the sulphur trioxide (SO3) molecule:

First, determine that there are twenty-four valence electrons in a single sulphur trioxide (SO3) molecule.

Determine how many additional valence electrons are required to complete the octet in the sulphur trioxide (SO3) molecule.

Six valence electrons are required to stabilise each sulphur and oxygen atom in a single sulphur trioxide (SO3) molecule.

Examine the number and nature of bonds that form within a single sulphur trioxide (SO3) molecule. There are three double covalent bonds between atoms of sulphur and oxygen.

Finally, locate the centre atom, which in this case is sulphur. Lastly, depict the skeleton as:

Why does SO3 create double bonds?

Due to the equal distribution of formal charge throughout the atom, double covalent bonds form in SO3.

It is defined by the number of electrons an atom contributed – the number of lone electron pairs – which is half the number of electrons in bond formation.

Therefore, for oxygen, 6-6-1 = 1

And the formula for sulphur is 6-0-3 = +3.

Now, the formal charge must be neutralised in order to produce a stable state, so +3 at the centre and -1 at the ends cannot exist. It is the reason why SO3 forms three double covalent bonds.

Chemical Structure of Sulfur Trioxide (SO3)

The graphic above demonstrates that the binding angle between oxygen-sulfur-oxygen (O-S-O) atoms must be more than 90 degrees.

In addition, the valence shell electron pair repulsion (VSEPR) hypothesis reveals that the structure of sulphur trioxide (SO3) is bent or trigonal pyramidal or trigonal planar, with a bond angle of 120 degrees.

As each oxygen atom forms a double bond with a sulphur atom and there is no lone pair on the centre atom (sulphur), there is no distortion in the bond angle.

In addition, there is an equal distribution of charges around the sulphur, which is why the sulphur required to expand its octet for this Lewis structure of SO3.

Consequently, the SO3 molecule is non-polar by nature. Examine the article concerning the polarity of SO3.

The Synthesis of Sulfur Trioxide (SO3)

The SO3 hybridization is sp2. It is calculated with the aid of a formula:

Number of hybrid orbitals equals the sum of the number of sigma bonds and lone pairs.

In a single shared double covalent bond, one sigma () bond and one pi () bond are present.

Therefore, there are three sigma bonds and zero lone pairs in a single SO3 molecule (confirm with the Lewis structure).

Thus, the number of hybrid orbitals equals 3 plus 0 In sp2 hybridization, one s orbital and two p orbitals of the same shell within an atom overlap and combine to form three new hybrid orbitals with comparable energy.

In addition, sp2 hybridization promotes trigonal symmetry with a bond angle of 120 degrees. In addition, these three new hybrid orbitals feature 33.33 percent s orbital traits and 66.66 percent p orbital characteristics.


Period 3 of the periodic table consists of elements whose octets tend to extend and accommodate more than eight valence electrons. It is interesting to note that this behaviour is not exceptional, as all elements save period two elements exhibit it. SO3 has sp2 hybridization due to the production of one sigma bond and one pi bond.

Read more: Polarity, Lewis Structure, Molecular Geometry, and Hybridization of SiO2

Misha Khatri
Misha Khatri is an emeritus professor in the University of Notre Dame's Department of Chemistry and Biochemistry. He graduated from Northern Illinois University with a BSc in Chemistry and Mathematics and a PhD in Physical Analytical Chemistry from the University of Utah.


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