Ch.10 - Chemical Bonding II: Molecular Shapes & Valence Bond Theory

Tro - Chemistry: A Molecular Approach 4th Edition

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Ch.10 - Chemical Bonding II: Molecular Shapes & Valence Bond Theory

Problem 75a,b

Chapter 10, Problem 75a,b

Using the molecular orbital energy ordering for second-row homonuclear diatomic molecules in which the π_2p orbitals lie at lower energy than the σ_2p, draw MO energy diagrams and predict the bond order in a molecule or ion with each number of total valence electrons. Will the molecule or ion be diamagnetic or paramagnetic? a. 4 b. 6

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Identify the second-row homonuclear diatomic molecule with 4 valence electrons. This could be a hypothetical molecule like Be2.

Construct the molecular orbital (MO) diagram for the molecule. For second-row diatomic molecules, the order of orbitals is: \( \sigma_{1s} \), \( \sigma^*_{1s} \), \( \sigma_{2s} \), \( \sigma^*_{2s} \), \( \pi_{2p} \), \( \sigma_{2p} \), \( \pi^*_{2p} \), \( \sigma^*_{2p} \).

Fill the molecular orbitals with the 4 valence electrons, starting from the lowest energy orbital. The electrons will fill the \( \sigma_{2s} \) and \( \sigma^*_{2s} \) orbitals.

Calculate the bond order using the formula: \( \text{Bond Order} = \frac{(\text{Number of bonding electrons} - \text{Number of antibonding electrons})}{2} \).

Determine if the molecule is diamagnetic or paramagnetic. A molecule is diamagnetic if all electrons are paired and paramagnetic if there are unpaired electrons.

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Key Concepts

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

Molecular Orbital Theory

Molecular Orbital Theory describes how atomic orbitals combine to form molecular orbitals, which can be occupied by electrons. In this theory, electrons are delocalized over the entire molecule, and the energy levels of these molecular orbitals determine the stability and properties of the molecule. Understanding the energy ordering of these orbitals is crucial for predicting bond order and magnetic properties.

Bond Order

Bond order is a measure of the number of chemical bonds between a pair of atoms, calculated as the difference between the number of bonding and antibonding electrons divided by two. A higher bond order indicates a stronger bond and greater stability of the molecule. For example, a bond order of 1 corresponds to a single bond, while a bond order of 2 corresponds to a double bond.

Diamagnetism and Paramagnetism

Diamagnetism and paramagnetism are terms used to describe the magnetic properties of substances based on their electron configurations. Diamagnetic substances have all paired electrons and are weakly repelled by a magnetic field, while paramagnetic substances have unpaired electrons and are attracted to a magnetic field. The presence of unpaired electrons in the molecular orbitals determines whether a molecule is diamagnetic or paramagnetic.

Related Practice

Textbook Question

Sketch the bonding and antibonding molecular orbitals that result from linear combinations of the 2p_z atomic orbitals in a homonuclear diatomic molecule. (The 2p_z orbitals are those whose lobes are oriented perpendicular to the bonding axis.) How do these molecular orbitals differ from those obtained from linear combinations of the 2p_y atomic orbitals? (The 2p_y orbitals are also oriented perpendicular to the bonding axis, but also perpendicular to the 2p_z orbitals.)

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Textbook Question

Using the molecular orbital energy ordering for second-row homonuclear diatomic molecules in which the π_2p orbitals lie at higher energy than the σ_2p, draw MO energy diagrams and predict the bond order in a molecule or ion with each number of total valence electrons. Will the molecule or ion be diamagnetic or paramagnetic? a. 10

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Textbook Question

Draw an MO energy diagram and predict the bond order of Li₂⁺ and Li₂^-. Do you expect these molecules to exist in the gas phase?

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Textbook Question

Draw an MO energy diagram and predict the bond order of Be₂⁺ and Be₂^- . Do you expect these molecules to exist in the gas phase?

Using the molecular orbital energy ordering for second-row homonuclear diatomic molecules in which the π_2p orbitals lie at lower energy than the σ_2p, draw MO energy diagrams and predict the bond order in a molecule or ion with each number of total valence electrons. Will the molecule or ion be diamagnetic or paramagnetic?? b. 6 d. 9

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