Ch.9 - Molecular Geometry and Bonding Theories

Brown - Chemistry: The Central Science 14th Edition

Brown14th EditionChemistry: The Central ScienceISBN: 9780134414232Not the one you use?Change textbook

Brown 14th Edition

Ch.9 - Molecular Geometry and Bonding Theories

Problem 14b

Chapter 9, Problem 14b

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

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Identify the molecular geometry of each molecule: PCl3 is trigonal pyramidal and ICl3 is T-shaped.

Understand the spatial arrangement of atoms in each geometry: Trigonal pyramidal has a three-dimensional structure with a central atom at the apex, whereas T-shaped is a planar geometry with three atoms in the same plane.

Recall that a molecule is considered flat if all its atoms lie in a single plane.

Compare the two geometries: Trigonal pyramidal has a vertical component due to the lone pair pushing down the bonding pairs, making it non-planar.

Conclude which molecule is flat by evaluating the planarity of the T-shaped geometry, which indicates that all atoms in ICl3 lie in the same plane.

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Key Concepts

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

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, which influence the shape. For example, PCl3 has a trigonal pyramidal shape due to one lone pair, while ICl3 has a T-shaped geometry due to three lone pairs.

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. According to VSEPR, electron pairs will arrange themselves to minimize repulsion, leading to specific molecular shapes. This theory helps explain why PCl3 is trigonal pyramidal and ICl3 is T-shaped.

Planarity in Molecules

Planarity in molecules refers to the arrangement of atoms in a single plane. A molecule is considered flat if all its atoms lie in the same plane, which is often the case for molecules with specific geometries like trigonal planar or linear. In this context, ICl3 is T-shaped and has a flat arrangement, while PCl3 is not flat due to its trigonal pyramidal shape.

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Related Practice

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.

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

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(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

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

Textbook Question

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

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Textbook Question

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