Ch.9 - Molecular Geometry and Bonding Theories

Brown - Chemistry: The Central Science 14th Edition

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Brown 14th Edition

Ch.9 - Molecular Geometry and Bonding Theories

Problem 14a

Chapter 9, Problem 14a

(a) Boron trichloride 1BCl32 and the carbonate ion 1CO3 2- 2 are both described as trigonal. What does this indicate about their bond angles?

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Trigonal geometry typically indicates that the molecule or ion has a central atom bonded to three other atoms or groups, forming a planar structure.

In a trigonal planar geometry, the bond angles are generally 120 degrees.

For boron trichloride (BCl3), the central boron atom is bonded to three chlorine atoms, forming a trigonal planar shape with bond angles of approximately 120 degrees.

The carbonate ion (CO3^{2-}) also has a trigonal planar shape, with the central carbon atom bonded to three oxygen atoms, resulting in bond angles of approximately 120 degrees.

Thus, both BCl3 and CO3^{2-} have bond angles close to 120 degrees due to their trigonal planar geometry.

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Key Concepts

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

Trigonal Geometry

Trigonal geometry refers to a molecular shape where a central atom is bonded to three other atoms, forming a planar structure. This arrangement typically results in bond angles of approximately 120 degrees, as seen in molecules like boron trichloride (BCl3) and the carbonate ion (CO3^2-). The trigonal shape arises from the repulsion between electron pairs around the central atom, which seeks to maximize distance between them.

Bond Angles

Bond angles are the angles formed between two adjacent bonds at a central atom. In trigonal planar molecules, the ideal bond angle is 120 degrees due to the symmetrical arrangement of the bonded atoms around the central atom. Variations in bond angles can occur due to factors such as lone pairs of electrons or differences in electronegativity among the bonded atoms, which can distort the ideal geometry.

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 VSEPR, electron pairs, whether bonding or non-bonding, will arrange themselves to minimize repulsion, leading to specific molecular shapes, such as trigonal planar for BCl3 and CO3^2-. This theory helps explain the observed bond angles in these molecules.

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(a) An AB2 molecule is linear. How many nonbonding electron pairs are around the A atom from this information?

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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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(b) How many nonbonding electrons surround the Xe in XeF2?

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Textbook Question

(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.

Textbook Question

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

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