Understanding Hybridisation of SO₃: Sulfur Trioxide

Sulfur Trioxide (SO₃) is also known as sulfuric anhydride. It is a molecule with a trigonal planar shape and does not contain any lone pair on the central atom - sulfur. It’s an excellent example of sp² hybridisation in inorganic chemistry.

Let us understand how hybridisation happens in SO₃. Read on to learn how it leads to its bonding and molecular shape.

What is the Hybridisation of SO₃?

Sulfur Trioxide consists of one sulfur atom and three oxygen atoms. Each oxygen atom forms a bond with sulfur; the S–O bonds are equivalent due to delocalised π electrons (resonance). In order to form these bonds and satisfy the octet rule, sulfur undergoes sp² hybridisation.

Breakdown of SO₃ Hybridisation

Sulfur trioxide is one of the most important oxides of sulfur. SO₃ is also used in producing sulphuric acid. It has a trigonal planar geometry along with equal bond lengths.

Here is a complete understanding of its hybridisation.

Electronic Configuration of Sulfur

The atomic number of sulfur is 16.

The ground state of sulfur :

1s² 2s² 2p⁶ 3s² 3p⁴

Only two unpaired electrons → insufficient to form three bonds

Excited state configuration:

1s² 2s² 2p⁶ 3s¹ 3p³ 3d¹

Three sp² orbitals form σ bonds with oxygen; the remaining unhybridised p orbital forms delocalised π bonds over all S–O bonds.

Ground state vs excited state orbital diagram

Formation of Hybrid Orbitals

sp² hybridisation occurs when 1 s orbital and 2 p orbitals mix.
The result:
→ 3 sp² hybrid orbitals on the sulfur atom
→ The remaining 1 unhybridised p orbital stays available for π bonding

Bond Formation in Sulfur Trioxide

The sulfur atom uses:

3 sp² orbitals to form a σ bond with oxygen
The unhybridised p orbital on sulfur overlaps with p orbitals on oxygen to form delocalised π bonds across all three S–O bonds, giving equivalent bond character.

Result:

3 S–O σ bonds
π bonds delocalised over the three S–O bonds
Hybridisation type: sp²
Bond angle: 120°
Geometry: Trigonal planar

Bonding of Sulfur Trioxide

Details At A Glance

Property	Details
Molecule	Sulfur Trioxide (SO₃)
Hybridisation	sp²
Geometry	Trigonal planar
Bond angle	120°
Bonding	3 σ bonds (S–O), 3 π bonds (delocalised)
Unhybridised Orbitals	1 (on sulfur for π bonding)
Sulfur valency satisfied?	Yes, each S–O bond contributes to octet; resonance delocalisation ensures equivalent bonding.

Formal Charge in SO₃

To determine if the Lewis structure of SO₃ is stable, we calculate the formal charge on each atom using the formula:

Formal charge = Valence electrons – (Lone pair electrons + ½ × Bonding electrons)

Step-by-step for each atom:

Sulfur (S) – central atom

Valence electrons: 6
Lone pairs: 0
Bonding electrons: 12
(from three double bonds with both O (presence of resonance))

Formal charge = 6 – (0 + ½×12) = 6 – 6 = 0

Oxygen (O) – each

Valence electrons: 6
Lone pairs: 2
Bonding electrons: 4
(1 double bond with sulfur)

Formal charge = 6 – (4 + ½×4) = 6 – (4 + 2) = 6 – (6) = 0

Thus, all atoms in SO₃ carry zero formal charge, confirming that the Lewis structure is stable and correct.

Summing Up

The sulfur in SO₃ forms 3 bonds with oxygen and shares 3 π bond (delocalised) with the same atoms. sp² hybridisation leads to a trigonal planar shape and 120° bond angles. And since the π bonds are delocalised, the bond length of S–O is equivalent.