Question

Describe the bonding in ${\mathrm{SO}}_2$ and ${\mathrm{SO}}_3$ using the localized electron model (hybrid orbital theory). How would the molecular orbital model describe the $\pi$ bonding in these two compounds?

Short answer

In both SO₂ and SO₃, sulfur is the central atom with trigonal planar electron domain geometry. In SO₂, sulfur undergoes sp² hybridization and forms two σ bonds with oxygen atoms, along with two π bonds created by overlapping p-orbitals. In SO₃, sulfur also undergoes sp² hybridization and forms three σ bonds with oxygen atoms, while the π electrons are delocalized over the entire molecule, resulting in resonance structures. In the molecular orbital model, the π bonding in SO₂ comprises one bonding π MO, one non-bonding MO, and one antibonding π* MO. In SO₃, the π bonding involves two bonding π MOs, one non-bonding MO, and one antibonding π* MO, accounting for delocalization and resonance.

Step-by-step solution

Electron Domain Geometry of SO₂

In the SO₂ molecule, the central atom S has 6 valence electrons, and each Oxygen atom contributes 6 valence electrons. Therefore, there are 18 valence electrons in total. The S atom forms two sigma bonds with the 2 O atoms and has one lone pair. So, there are three electron domains, and according to the VSEPR model, the electron domain geometry is trigonal planar.

Electron Domain Geometry of SO₃

In the SO₃ molecule, the central atom S has 6 valence electrons, and each Oxygen atom contributes 6 valence electrons. Therefore, there are 24 valence electrons in total. The S atom forms three sigma bonds with the 3 O atoms. So, there are three electron domains, and according to the VSEPR model, the electron domain geometry is trigonal planar. Step 2: Determining the Hybridization of the Central Atom

Hybridization of S in SO₂

Since the electron domain geometry of SO₂ is trigonal planar, the central atom S undergoes sp² hybridization, forming 3 sp² hybrid orbitals.

Hybridization of S in SO₃

Since the electron domain geometry of SO₃ is trigonal planar, the central atom S undergoes sp² hybridization, forming 3 sp² hybrid orbitals. Step 3: Describing the π Bonding using the Localized Electron Model

π Bonding in SO₂

In SO₂, one unhybridized p-orbital from the S and each of the two O atoms are involved in the π bonding. These p-orbitals overlap side-by-side, forming two π bonds.

π Bonding in SO₃

In SO₃, one unhybridized p-orbital from the S and each of the three O atoms contribute to the π bonding. The π electrons are delocalized over the entire molecule, resulting in resonance structures with one double bond and two single bonds between S and the O atoms. Step 4: Describing the π Bonding in the Molecular Orbital Model

Molecular Orbital Description of π Bonding in SO₂

In the molecular orbital model, the π bonding in SO₂ is described as the overlap of the three p-orbitals (one from S and two from O atoms) to form three molecular orbitals: one bonding π MO (occupied by two electrons), one non-bonding MO (occupied by two electrons), and one antibonding π* MO (occupied by zero electrons).

Molecular Orbital Description of π Bonding in SO₃

In the molecular orbital model, the π bonding in SO₃ is described as the overlap of four p-orbitals (one from S and three from O atoms) to form four molecular orbitals: two bonding π MOs (each occupied by two electrons), one non-bonding MO (occupied by two electrons), and one antibonding π* MO (occupied by zero electrons). This accounts for the delocalization and resonance observed in the SO₃ molecule.