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Ch.9 - Molecular Geometry and Bonding Theories
All textbooksBrown 14th EditionCh.9 - Molecular Geometry and Bonding TheoriesProblem 36
Chapter 9, Problem 36

Consider a molecule with the formula AX2. Supposing the A-X bond is polar, how would you expect the dipole moment of the AX2 molecule to change as the X-A-X bond angle decreases from 180° to 100°?

Verified step by step guidance
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Step 1: Understand the concept of dipole moment. The dipole moment is a measure of the separation of positive and negative charges in a molecule. It is a vector quantity, meaning it has both magnitude and direction.
Step 2: Consider the geometry of the AX2 molecule. When the X-A-X bond angle is 180°, the molecule is linear, and if the A-X bonds are equally polar, their dipole moments cancel each other out, resulting in a net dipole moment of zero.
Step 3: Analyze the effect of decreasing the bond angle. As the X-A-X bond angle decreases from 180° to 100°, the molecule becomes bent. This change in geometry affects the vector addition of the individual bond dipoles.
Step 4: Predict the change in dipole moment. With a bent geometry, the dipole moments of the A-X bonds no longer cancel completely. The net dipole moment becomes non-zero and increases as the bond angle decreases.
Step 5: Conclude the relationship. The dipole moment of the AX2 molecule increases as the X-A-X bond angle decreases from 180° to 100°, due to the change from a linear to a bent molecular shape, which prevents complete cancellation of the bond dipoles.

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. For a molecule with the formula AX2, the geometry can be linear at 180° or bent as the bond angle decreases. The shape influences how dipoles from polar bonds combine, affecting the overall dipole moment.
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Dipole Moment

The dipole moment is a measure of the separation of positive and negative charges in a molecule, represented as a vector quantity. It depends on both the magnitude of the charge and the distance between the charges. In polar molecules, the dipole moment can change with bond angles, as the orientation of the dipoles relative to each other alters the net dipole.
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Bond Angle and Polarity

The bond angle in a molecule affects the vector sum of individual bond dipoles. As the bond angle decreases from 180° to 100°, the individual dipole moments of the A-X bonds do not cancel out as effectively, leading to an increase in the overall dipole moment. This change illustrates how molecular shape and bond angles influence molecular polarity.
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Related Practice
Textbook Question

(b) How would you expect the H¬X¬H bond angle to vary in the series H2O, H2S, H2Se? Explain. (Hint: The size of an electron pair domain depends in part on the electronegativity of the central atom.)

Textbook Question

(a) Does CS2 have a dipole moment? If so, in which direction does the net dipole point? (b) Does SO2 have a dipole moment? If so, in which direction does the net dipole point?

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Textbook Question

(b) It turns out that ozone, O3, has a small dipole moment. How is this possible, given that all the atoms are the same?

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