Ch.9 - Molecular Geometry and Bonding Theories

Problem 104d

Chapter 9, Problem 104d

One of the molecular orbitals of the H2− ion can be sketched as follows:
d. Compared to the H—H bond in H2, the H—H bond in H2− is expected to be which of the following?
i. shorter and stronger
ii. longer and stronger
iii. shorter and weaker
iv. longer and weaker or
v. the same length and strength

Verified step by step guidance

insert step 1> Consider the molecular orbital theory for diatomic molecules, which explains bonding in terms of the combination of atomic orbitals to form molecular orbitals.

insert step 2> For the H2 molecule, there are two electrons that fill the bonding molecular orbital, resulting in a stable bond.

insert step 3> In the H2− ion, there is an additional electron, which occupies the antibonding molecular orbital.

insert step 4> The presence of an electron in the antibonding orbital reduces the overall bond order, which is calculated as (number of electrons in bonding orbitals - number of electrons in antibonding orbitals) / 2.

insert step 5> A reduced bond order typically results in a longer and weaker bond compared to the original H2 molecule.

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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 are formed by the combination of atomic orbitals when atoms bond together. In the case of the H2− ion, the additional electron contributes to the formation of a bonding molecular orbital, which can enhance the stability of the molecule. Understanding how these orbitals interact is crucial for predicting bond characteristics such as length and strength.

Bond Length and Strength

Bond length refers to the distance between the nuclei of two bonded atoms, while bond strength indicates the energy required to break that bond. Generally, shorter bonds are stronger due to the increased overlap of atomic orbitals, which leads to a more stable interaction. Analyzing how the presence of an extra electron in H2− affects these properties is essential for answering the question.

Electron Repulsion and Stability

In molecular systems, electron-electron repulsion can influence the stability and characteristics of a bond. The addition of an extra electron in H2− can lead to increased repulsion between electrons, potentially weakening the bond compared to H2. Understanding this concept helps in evaluating how the H—H bond in H2− differs from that in H2.

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Related Practice

Textbook Question

The structure of borazine, B₃N₃H₆, is a six-membered ring of alternating B and N atoms. There is one H atom bonded to each B and to each N atom. The molecule is planar. (e) What are the hybridizations at the B and N atoms in the Lewis structures from parts (a) and (b)? Would you expect the molecule to be planar for both Lewis structures? Would you expect the molecule to be planar for both Lewis structures?

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Textbook Question

The highest occupied molecular orbital of a molecule is abbreviated as the HOMO. The lowest unoccupied molecular orbital in a molecule is called the LUMO. Experimentally, one can measure the difference in energy between the HOMO and LUMO by taking the electronic absorption (UV-visible) spectrum of the molecule. Peaks in the electronic absorption spectrum can be labeled as p2p9p2p*, s2s9s2s*, and so on, corresponding to electrons being promoted from one orbital to another. The HOMO-LUMO transition corresponds to molecules going from their ground state to their first excited state. (c) The electronic absorption spectrum of the N2 molecule has the lowest energy peak at 170 nm. To what orbital transition does this correspond?

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Textbook Question

One of the molecular orbitals of the H2− ion can be sketched as follows:

a. Is the molecular orbital a 𝜎 or 𝜋 MO? Is it bonding or antibonding?

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Textbook Question

Place the following molecules and ions in order from smallest to largest bond order: H2+,B2,N2+,F2+, and Ne2.

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Textbook Question

Azo dyes are organic dyes that are used for many applications, such as the coloring of fabrics. Many azo dyes are derivatives of the organic substance azobenzene, C12H10N2. A closely related substance is hydrazobenzene, C12H12N2. The Lewis structures of these two substances are

(Recall the shorthand notation used for benzene.) (b) How many unhybridized atomic orbitals are there on the N and the C atoms in each of the substances? How many unhybridized atomic orbitals are there on the N and the C atoms in hydrazobenzene?

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Textbook Question

(Recall the shorthand notation used for benzene.) (c) Predict the N¬N¬C angles in each of the substances.

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