Ch.9 - Molecular Geometry and Bonding Theories

Problem 109b

Chapter 9, Problem 109b

The energy-level diagram in Figure 9.40 shows that the sideways overlap of a pair of p orbitals produces two molecular orbitals, one bonding and one antibonding. In ethylene there is a pair of electrons in the bonding orbital between the two carbons. Absorption of a photon of the appropriate wavelength can result in promotion of one of the bonding electrons from the to the molecular orbital. b. Assuming this electronic transition corresponds to the HOMO–LUMO transition, what is the LUMO in ethylene?

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Identify the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO) in ethylene. The HOMO is the highest energy orbital that contains electrons, while the LUMO is the lowest energy orbital that is empty.

Understand that in ethylene, the HOMO is the bonding \\(\pi\\) orbital formed by the sideways overlap of the p orbitals on each carbon atom.

Recognize that the LUMO in ethylene is the antibonding \\(\pi^*\\) orbital, which is the next available orbital above the HOMO that electrons can be promoted to upon absorption of energy.

Relate the electronic transition from the HOMO to the LUMO to the absorption of a photon. This transition involves an electron in the bonding \\(\pi\\) orbital being excited to the antibonding \\(\pi^*\\) orbital.

Conclude that the LUMO in ethylene, considering the HOMO–LUMO transition, is the antibonding \\(\pi^*\\) orbital.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Molecular Orbitals

Molecular orbitals (MOs) are formed from the linear combination of atomic orbitals (LCAO) when atoms bond. In ethylene, the sideways overlap of p orbitals creates two types of MOs: bonding and antibonding. The bonding molecular orbital is lower in energy and stabilizes the molecule, while the antibonding orbital is higher in energy and destabilizes it. Understanding these orbitals is crucial for analyzing electronic transitions in molecules.

HOMO and LUMO

The Highest Occupied Molecular Orbital (HOMO) is the molecular orbital that contains the highest energy electrons in a molecule, while the Lowest Unoccupied Molecular Orbital (LUMO) is the lowest energy orbital that is unoccupied. In ethylene, the transition of an electron from the HOMO to the LUMO upon photon absorption is fundamental to understanding its electronic properties and reactivity. Identifying these orbitals helps predict how the molecule will interact with light.

Electronic Transitions

Electronic transitions refer to the movement of electrons between different energy levels or orbitals within a molecule, typically induced by the absorption of light. In the context of ethylene, when a photon of the appropriate wavelength is absorbed, an electron can be promoted from the bonding orbital (HOMO) to the antibonding orbital (LUMO). This transition is essential for understanding the absorption spectrum and the photochemical behavior of the molecule.

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The energy-level diagram in Figure 9.40 shows that the sideways overlap of a pair of p orbitals produces two molecular orbitals, one bonding and one antibonding. In ethylene there is a pair of electrons in the bonding orbital between the two carbons. Absorption of a photon of the appropriate wavelength can result in promotion of one of the bonding electrons from the to the molecular orbital. a. Assuming this electronic transition corresponds to the HOMO–LUMO transition, what is the HOMO in ethylene?

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Textbook Question

The energy-level diagram in Figure 9.40 shows that the sideways overlap of a pair of p orbitals produces two molecular orbitals, one bonding and one antibonding. In ethylene there is a pair of electrons in the bonding orbital between the two carbons. Absorption of a photon of the appropriate wavelength can result in promotion of one of the bonding electrons from the to the molecular orbital. c. Is the bond in ethylene stronger or weaker in the excited state than in the ground state? Why?

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Sulfur tetrafluoride (SF₄) reacts slowly with O₂ to form sulfur tetrafluoride monoxide (OSF₄) according to the following unbalanced reaction: SF₄(g) + O₂(g) → OSF₄(g) The O atom and the four F atoms in OSF₄ are bonded to a central S atom. (b) Write a Lewis structure of OSF₄ in which the formal charges of all atoms are zero.

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