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SO3 Molecular Geometry

We know that atoms and molecules are basic of matter. Here atoms are the simplest unit of any matter which can form chemical bonds with similar or different atoms to form molecules. The chemical bonds can be Ionic or covalent type which depends upon the nature of atoms bonded in the molecule. Ionic molecules are composed of cations and anions which are bonded with each other through the electrostatic force of attraction to each other.

SO3 Structure
On the contrary, in covalent bond the atoms are bonded with each other through equal sharing of electrons between them. In the covalent molecules, the atoms are arranged in certain geometry with fixed bond angel and bond length. Valence bond theory of chemical bonding explains the bonding and geometry of molecules. According to VSEPR theory, the presence of lone pair alters the geometry of molecules from its regular geometry. Due to this the bond angle and bond length also. The bond pairs of bonded atoms and lone pairs of bonded atoms are arranged in such a way that there is the maximum distance between them for minimum repulsion so that a molecule will be more stabilise. Valence bond theory is based on the concept of hybridisation and resonance. Let’s have a look on the molecular geometry of sulphur trioxide on the basis of resonance.

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Resonance Structure of SO3

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The SO3 molecule can be represented as three different electron dot structures which are referred to as being in resonance. These three structures are called resonance structures. The inclusion of double arrows is often used to represent resonance.
Resonance results from electrons moving around the entire molecule instead of pairing. The SO3 structure is a single structure which can only be represented as an average of the various resonance structures.
A method of indicating resonance is to condense the various forms into a single structure which uses a broken line to indicate that the S-O bonds are all equivalent.

SO3 Resonance
Resonance is an important characteristic of organic compounds especially those based on the benzene ring and is a factor involved in determining the shape of a structure.

Bond Angles of SO3

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Sulfur trioxide is regarded as an anhydride of sulfuric acid (H2SO4). SO3 has a tirgonal planar structure with bond angle 120o. The representation of SO3 molecule is shown below.
Bond Angle SO3

Sulfur trioxide symmetry is restored and the trigonal-planar shape is obtained. VSEPR is a simple method for predicting the shapes of small molecules containing a central atom. At room temperature SO3 is a solid and exists in three forms. Gamma-SO3 is ice like and is a cyclic trimer. Beta-SO3 is made up of infinite helical chains of tetrahedral [SO4] units each sharing two corners. Alpha-SO3 is made up of chains cross linked into sheets.
Cyclic Trimer

Hybridization of SO3

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In gas phase SO3 molecule is triangular planar involving sp2 hybridization of the S atom. There are three sigma bonds formed by sp2-p overlap and three pi bonds, one by p(pi)-p(pi) overlap and two by p(pi)-d(pi) overlap. O-S-O bond angles is 120o

Hybridization of SO3

One s and two p orbitals of one atom hybridize to give three equivalent sp2 hybrid orbitals. These three sp2 hybrid orbitals are directed towards the three corners of an equivalent triangle with an angle of 120o and give a triangular geometry to the molecule. sp2 hybrid orbitals are larger in size than sp hybrid orbitals but slightly smaller than that of sp3 hybrid orbitals. Each sp3 hybrid orbitals has   $\frac{1}{3}$ s character and $\frac{2}{3}  $ p character.

Molecular Geometry Chart

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The molecular geometry of SO3 is trigonal planar. It acts as a good oxidizing agent. The molecular geometry chart for sulfur trioxide molecule is shown below.

Number of Bonds Geometry
Type of Hybrid orbital
Angle between bonded atoms Example
6 Trigonal planar sp2 120o

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