Ch.9 - Molecular Geometry and Bonding Theories

Problem 112b

Chapter 9, Problem 112b

The energy-level diagram in Figure 9.36 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 π 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 p2p to the p*2p 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) in ethylene, which is the bonding π orbital formed by the sideways overlap of p orbitals.

Determine the Lowest Unoccupied Molecular Orbital (LUMO) in ethylene, which is the antibonding π* orbital formed by the sideways overlap of p orbitals.

Understand that the HOMO-LUMO transition involves an electron being excited from the HOMO (bonding π orbital) to the LUMO (antibonding π* orbital).

Recognize that the LUMO in ethylene is the π*2p molecular orbital.

Conclude that the LUMO in ethylene is the antibonding π* orbital formed by the sideways overlap of the p orbitals.

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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 when atomic orbitals combine during the bonding process. In the case of ethylene, the sideways overlap of p orbitals creates two types of MOs: bonding and antibonding. The bonding molecular orbital (π) is lower in energy and stabilizes the molecule, while the antibonding molecular orbital (π*) 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) and the Lowest Unoccupied Molecular Orbital (LUMO) are key concepts in molecular orbital theory. The HOMO is the highest energy orbital that contains electrons, while the LUMO is the lowest energy orbital that is empty. The transition of an electron from the HOMO to the LUMO upon photon absorption is fundamental in understanding the electronic properties of molecules, such as their reactivity and color.

Electronic Transitions

Electronic transitions refer to the movement of electrons between different energy levels or orbitals within a molecule. In ethylene, the absorption of a photon can promote an electron from the bonding π orbital (HOMO) to the antibonding π* orbital (LUMO). This transition is significant in spectroscopy and photochemistry, as it can lead to various chemical reactions and changes in molecular properties, such as color and stability.

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Textbook Question

(c) It turns out that the difference in energies between the valence atomic orbitals of H and F are sufficiently different that we can neglect the interaction of the 1s orbital of hydrogen with the 2s orbital of fluorine.

The 1s orbital of hydrogen will mix only with one 2p orbital of fluorine. Draw pictures showing the proper orientation of all three 2p orbitals on F interacting with a 1s orbital on H. Which of the 2p orbitals can actually make a bond with a 1s orbital, assuming that the atoms lie on the z-axis?

Textbook Question

Carbon monoxide, CO, is isoelectronic to N2. (d) Would you expect the p2p MOs of CO to have equal atomic orbital contributions from the C and O atoms? If not, which atom would have the greater contribution?

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Textbook Question

The energy-level diagram in Figure 9.36 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 π 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 p2p to the p*2p 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 9.36 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 π 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 p2p to the p*2p molecular orbital. (c) Is the C¬C bond in ethylene stronger or weaker in the excited state than in the ground state? Why?

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Open Question

A compound composed of 6.7% H, 40.0% C, and 53.3% O has a molar mass of approximately 60 g>mol. (c) What is the geometry and hybridization of the C atom that is bonded to 2 O atoms?

Open Question

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. (d) Determine the electron-domain geometry of OSF₄, and write two possible molecular geometries for the molecule based on this electron-domain geometry. (e) For each of the molecules you drew in part (d), state how many fluorines are equatorial and how many are axial.