Book cover for Chemistry: The Molecular Nature of Matter

Chemistry: The Molecular Nature of Matter

Neil D. Jespersen, James E. Brady, Alison Hyslop

ISBN #9781118413920

7th Edition

3,064 Questions

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53,557 Students Helped

Homework Questions

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Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

This chapter provides an integrated approach to understanding chemical bonding through multiple theories. It emphasizes that molecular geometry arises from both electron pair repulsions (VSEPR) and orbital hybridizations (Valence Bond theory), while Molecular Orbital theory offers insights into electron delocalization, bond order, and the properties of diatomic molecules and solids. Together, these theories form a comprehensive framework for analyzing molecular shapes, bond formation, and the properties of materials.

Learning Objectives

1

Describe and compare the different bonding theories (VSEPR, Valence Bond, and Molecular Orbital) and their roles in determining molecular geometry and chemical properties.

2

Explain how electron pair repulsions and orbital hybridizations influence molecular shapes.

3

Analyze the application of VSEPR in predicting molecular shapes and examine the directional nature of covalent bonds using Valence Bond theory.

4

Evaluate electron delocalization in molecules and solids through the principles of Molecular Orbital theory.

Key Concepts

CONCEPT

DEFINITION

VSEPR Model

A model that uses electron pair repulsions to predict the geometry of molecules.

Valence Bond (VB) Theory

A theory that explains the formation of covalent bonds through orbital overlap and hybridization, emphasizing the directional nature of bonds.

Molecular Orbital (MO) Theory

A theory that describes how atomic orbitals combine to form molecular orbitals that are delocalized over the entire molecule, determining bond order and electronic properties.

Orbital Hybridization

The process of mixing atomic orbitals to form new hybrid orbitals that can explain molecular geometry and bond angles.

Dipole Moment

A measure of the separation of positive and negative charges in a molecule that results from its molecular geometry.

Example Problems

Example 1

Sketch the following molecular shapes and give the various bond angles in the structures: (a) planar triangular, (b) tetrahedral, (c) octahedral.

Example 2

Sketch the following molecular shapes and give the bond angles in the structures: (a) linear, (b) trigonal bipyramidal.

Example 3

What is the underlying principle on which the VSEPR model is based?

Example 4

What is an electron domain? How are nonbonding and double bonds described by electron domains?

Example 5

How many bonding domains and how many nonbonding domains are there in a molecule of formaldehyde, HCHO?
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Step-by-Step Explanations

QUESTION

How does the VSEPR model predict the shape of a molecule such as water (H2O)?

STEP-BY-STEP ANSWER:

Step 1: Identify the central atom and count the total number of electron pairs (bonding and lone pairs) around it. For H2O, the oxygen has 2 bonding pairs and 2 lone pairs.
Step 2: Arrange the electron pairs to minimize repulsion. With four electron pairs, a tetrahedral electron geometry is assumed.
Step 3: Account for the lone pairs which occupy more space than bonding pairs, leading to a bent or V-shaped molecular geometry.
Final Answer: The water molecule has a bent molecular shape due to the two lone pairs on the oxygen atom.

VSEPR Model

QUESTION

How does hybridization explain the tetrahedral shape observed in methane (CH4)?

STEP-BY-STEP ANSWER:

Step 1: Determine the number of electron domains around the central atom (carbon in CH4 has four electron domains, all bonding pairs).
Step 2: Recognize that carbon undergoes sp3 hybridization, mixing one s orbital and three p orbitals to form four equivalent sp3 hybrid orbitals.
Step 3: Arrange the sp3 hybrid orbitals in a tetrahedral geometry to minimize repulsion, allowing for equal bond angles of 109.5°.
Final Answer: Hybridization in methane results in sp3 orbitals arranged tetrahedrally, accounting for its shape.

Hybridization in Valence Bond Theory

QUESTION

How does MO theory explain the bond order in a simple diatomic molecule like O2?

STEP-BY-STEP ANSWER:

Step 1: Combine the atomic orbitals of the two oxygen atoms to form molecular orbitals, including bonding and antibonding orbitals.
Step 2: Fill the molecular orbitals with the total electrons from both atoms following the Aufbau principle and Hund's rule.
Step 3: Calculate bond order using the formula: Bond Order = ½(Number of electrons in bonding orbitals − Number of electrons in antibonding orbitals).
Step 4: For O2, this results in a bond order of 2, which corresponds to a double bond.
Final Answer: MO theory predicts a bond order of 2 for O2, indicating a double bond based on the electron configuration in the molecular orbitals.

Molecular Orbital Theory

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Common Mistakes

  • Confusing the purpose of each bonding theory and applying their principles interchangeably without recognizing their distinct assumptions.
  • Overlooking the impact of lone pairs on molecular geometry when using the VSEPR model.
  • Misidentifying the type of hybridization occurring in molecules, leading to incorrect predictions of molecular shape.
  • Neglecting the difference between localized electron pairs (as in VB theory) and delocalized electrons (as in MO theory) in explaining chemical properties.