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Ch.9 - Molecular Geometry and Bonding Theories
All textbooksBrown 14th EditionCh.9 - Molecular Geometry and Bonding TheoriesProblem 17a,c
Chapter 9, Problem 17a,c

(a) An AB6 molecule has no lone pairs of electrons on the A atom. What is its molecular geometry? (c) For the AB4 molecule in part (b), predict the molecular geometry.

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Step 1: Understand the concept of molecular geometry. Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. It is determined by the number of bonding pairs and lone pairs of electrons around the central atom.
Step 2: Analyze the AB6 molecule. Since the A atom has no lone pairs and is surrounded by six B atoms, the electron pairs are arranged to minimize repulsion. This arrangement is typically octahedral, where the bond angles are 90 degrees.
Step 3: Consider the AB4 molecule. If the A atom has no lone pairs and is surrounded by four B atoms, the electron pairs will arrange themselves in a way that minimizes repulsion, typically resulting in a tetrahedral geometry with bond angles of approximately 109.5 degrees.
Step 4: Apply VSEPR theory. VSEPR (Valence Shell Electron Pair Repulsion) theory helps predict the geometry of molecules based on the repulsion between electron pairs. For AB6, the octahedral shape is predicted, and for AB4, the tetrahedral shape is predicted.
Step 5: Visualize the molecular geometries. For AB6, imagine a central atom with six surrounding atoms forming an octahedron. For AB4, visualize a central atom with four surrounding atoms forming a tetrahedron.

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Key Concepts

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

VSEPR Theory

Valence Shell Electron Pair Repulsion (VSEPR) Theory is a model used to predict the geometry of molecules based on the repulsion between electron pairs surrounding a central atom. According to this theory, electron pairs, whether bonding or lone pairs, will arrange themselves to minimize repulsion, leading to specific molecular shapes.
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Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule. It is determined by the number of bonding pairs and lone pairs of electrons around the central atom, which influences the overall shape, such as linear, trigonal planar, tetrahedral, octahedral, etc.
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Coordination Number

The coordination number is the number of atoms, ions, or molecules that a central atom can bond with in a molecule. In the case of AB<sub>6</sub>, the coordination number is six, indicating that the central atom A is bonded to six B atoms, which typically results in an octahedral geometry.
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Related Practice
Textbook Question

(b) The PCl3 molecule is trigonal pyramidal, while ICl3 is T-shaped. Which of these molecules is flat?

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Textbook Question
How does a trigonal pyramid differ from a tetrahedron so far as molecular geometry is concerned?
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Textbook Question

Describe the bond angles to be found in each of the following molecular structures: (a) trigonal planar, (b) tetrahedral, (c) octahedral, (d) linear.

Textbook Question

(b) An AB4 molecule has two lone pairs of electrons on the A atom (in addition to the four B atoms). What is the electron-domain geometry around the A atom?

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Textbook Question

Would you expect the nonbonding electron-pair domain in NCl3 to be greater or smaller in size than the corresponding one in PCl3?

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