Ch.9 - Molecular Geometry and Bonding Theories

Brown - Chemistry: The Central Science 15th Edition

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Brown 15th Edition

Ch.9 - Molecular Geometry and Bonding Theories

Problem 16a

Chapter 9, Problem 16a

a. Methane (CH4) and the perchlorate ion (ClO4−) are both described as tetrahedral. What does this indicate about their bond angles?

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Tetrahedral geometry is a common molecular shape in chemistry, characterized by four groups of electrons around a central atom.

In a tetrahedral geometry, the bond angles are determined by the repulsion between electron pairs, which is explained by the VSEPR (Valence Shell Electron Pair Repulsion) theory.

According to VSEPR theory, the electron pairs will arrange themselves as far apart as possible to minimize repulsion, resulting in a bond angle of approximately 109.5 degrees.

For methane (CH4), the central carbon atom is bonded to four hydrogen atoms, forming a perfect tetrahedral shape with bond angles of 109.5 degrees.

Similarly, in the perchlorate ion (ClO4−), the central chlorine atom is surrounded by four oxygen atoms, also forming a tetrahedral shape with bond angles of approximately 109.5 degrees.

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Key Concepts

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

Tetrahedral Geometry

Tetrahedral geometry occurs when a central atom is bonded to four other atoms, resulting in a three-dimensional shape. In this arrangement, the bond angles are approximately 109.5 degrees, which minimizes electron pair repulsion according to VSEPR (Valence Shell Electron Pair Repulsion) theory. This geometry is characteristic of molecules like methane (CH4) and the perchlorate ion (ClO4−).

Bond Angles

Bond angles are the angles formed between adjacent bonds in a molecule. In tetrahedral molecules, the ideal bond angle is about 109.5 degrees due to the spatial arrangement of the electron pairs around the central atom. Deviations from this angle can occur due to factors such as lone pairs or differences in electronegativity among bonded atoms.

VSEPR Theory

VSEPR theory is a model used to predict the geometry of molecules based on the repulsion between electron pairs surrounding a central atom. It posits that electron pairs will arrange themselves to be as far apart as possible, leading to specific molecular shapes, such as tetrahedral for molecules with four bonding pairs. This theory helps explain the bond angles observed in molecules like CH4 and ClO4−.

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(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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b. The NH3 molecule is trigonal pyramidal, while BF3 is trigonal planar. Which of these molecules is flat?

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For each of these contour representations of molecular orbitals, identify (c) whether the MO is bonding or antibonding (i)

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

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For each of these contour representations of molecular orbitals, identify (b) the type of MO (s or p) (iii)

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For each of these contour representations of molecular orbitals, identify (b) the type of MO (s or p) (i)

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