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Ch. 1 - Remembering General Chemistry: Electronic Structure and Bonding (Part 2)
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook
All textbooksBruice 8th EditionCh. 1 - Remembering General Chemistry: Electronic Structure and Bonding (Part 2)Problem 47
Chapter 2, Problem 47

Account for the difference in the shape and color of the potential maps for ammonia and the ammonium ion in Section 1.11.
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Ammonia (NH₃) has a trigonal pyramidal geometry due to the lone pair of electrons on the nitrogen atom. This lone pair creates a region of high electron density, which is reflected in the potential map as a region of negative electrostatic potential (typically shown in red). The three hydrogen atoms bonded to nitrogen contribute to regions of lower electron density (positive potential, typically shown in blue).
The ammonium ion (NH₄⁺), on the other hand, has a tetrahedral geometry. In this ion, the lone pair on nitrogen is replaced by a fourth hydrogen atom, resulting in a symmetrical distribution of electron density. The absence of a lone pair means there is no region of high negative potential, and the electrostatic potential map will show a more uniform distribution of positive potential around the molecule.
The difference in shape between ammonia and the ammonium ion arises from the presence or absence of the lone pair on nitrogen. In ammonia, the lone pair causes a distortion in geometry, while in ammonium, the geometry is symmetrical due to the equal distribution of bonding electrons.
The color difference in the potential maps is due to the change in electron density. Ammonia's map will show a distinct region of negative potential (red) due to the lone pair, while ammonium's map will lack this feature and instead show a more uniform positive potential (blue or green).
These differences in shape and color are directly related to the electronic structure and bonding of the two species. Ammonia's lone pair contributes to its polarity and uneven charge distribution, while ammonium's symmetrical structure results in a more even charge distribution.

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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 influences the shape and angles between bonds. For ammonia (NH3), the geometry is trigonal pyramidal, while the ammonium ion (NH4+) has a tetrahedral shape, affecting their potential maps.
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Electronegativity and Dipole Moments

Electronegativity is the tendency of an atom to attract electrons in a bond, leading to the formation of dipole moments. In ammonia, the nitrogen atom is more electronegative than hydrogen, creating a polar molecule with a significant dipole moment. In contrast, the ammonium ion has a symmetrical charge distribution, resulting in no net dipole moment, which influences the shape and color of their potential maps.
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Potential Energy Maps

Potential energy maps visually represent the energy landscape around a molecule, indicating regions of high and low potential energy based on electron density and molecular interactions. The shape and color variations in these maps reflect the distribution of electron density and the stability of different molecular conformations, which differ between ammonia and the ammonium ion due to their distinct geometries and charge distributions.
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Related Practice
Textbook Question

If the dipole moment of CH3F is 1.847 D and the dipole moment of CD3F is 1.858 D, which is more electronegative: hydrogen or deuterium?

Textbook Question

Which of the following has a bond closest to the ionic end of the bond spectrum?

CH3NH2 CH3CH3 CH3F CH3OH

3
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Textbook Question

Which of the following molecules would you expect to have a dipole moment of zero?

a. CH3CH3

b. H2C═O

c. CH2Cl2

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Textbook Question

Which of the following has a polar covalent bond?

CH3NH2 CH3CH3 CH3F CH3OH

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Textbook Question

Which of the following molecules would you expect to have a dipole moment of zero?

d. NH3

e. H2C═CH2

f. H2C═CHBr

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Textbook Question

Which of the following molecules would you expect to have a dipole moment of zero?

g. BeCl2

h. BF3

7
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