One of the molecular orbitals of the H2- ion is sketched below: (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?

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Identify the nature of the molecular orbital in H2- compared to H2. H2- has an extra electron compared to H2, which occupies an antibonding orbital.

Understand the effect of an electron in an antibonding orbital. Electrons in antibonding orbitals decrease the bond order and weaken the bond between atoms.

Recall the relationship between bond order, bond length, and bond strength. A lower bond order typically results in a longer and weaker bond.

Apply the concept to the H2- ion. Since the extra electron in H2- is in an antibonding orbital, it reduces the bond order, leading to a weaker and longer bond compared to H2.

Conclude the expected characteristics of the H-H bond in H2- based on the presence of the electron in the antibonding orbital. The bond in H2- is expected to be longer and weaker compared to the bond in H2.

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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 influence bond length and strength is crucial for analyzing the properties of diatomic molecules.

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. In the context of H2 and H2-, the presence of an extra electron affects both the bond length and strength.

Electron Repulsion

In molecular systems, electron-electron repulsion can influence the geometry and stability of a molecule. In H2-, the additional electron increases repulsion between electrons, which can lead to a longer bond length compared to H2. This concept is essential for predicting how the presence of extra electrons in an anion like H2- alters the characteristics of the bond compared to the neutral H2 molecule.

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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 π_2p-π_2p*, σ_s-σ_2s*, 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?

Textbook Question

One of the molecular orbitals of the H2- ion is sketched below:

(a) Is the molecular orbital a s or p MO? Is it bonding or antibonding?

Textbook Question

Place the following molecules and ions in order from smallest to largest bond order: N22+, He2+, Cl2 H2-, O22-.

Textbook Question

Molecules that are brightly colored have a small energy gap between filled and empty electronic states (the HOMOLUMO gap; see Exercise 9.104). Suppose you have two samples, one is lycopene which is responsible for the red color in tomato, and the other is curcumin which is responsible for the yellow color in turmeric. Which one has the larger HOMO-LUMO gap?

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?