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 31

Chapter 9, Problem 31

Ammonia, NH3, reacts with incredibly strong bases to produce the amide ion, NH2-. Ammonia can also react with acids to produce the ammonium ion, NH4+. (a) Which species (amide ion, ammonia, or ammonium ion) has the largest H¬N¬H bond angle? (b) Which species has the smallest H¬N¬H bond angle?

Verified step by step guidance

Understand the molecular geometry of each species: Ammonia (NH3) is a trigonal pyramidal molecule, Ammonium ion (NH4+) is a tetrahedral molecule, and Amide ion (NH2-) is a bent molecule.

Recall that the bond angle in a tetrahedral molecule is approximately 109.5 degrees, in a trigonal pyramidal molecule it is slightly less due to lone pair repulsion, and in a bent molecule it is even smaller.

Compare the bond angles: The tetrahedral geometry of NH4+ will have the largest bond angle, approximately 109.5 degrees.

Identify the species with the smallest bond angle: The bent geometry of NH2- will have the smallest bond angle, typically less than 109.5 degrees due to increased lone pair repulsion.

Conclude that NH4+ has the largest H-N-H bond angle and NH2- has the smallest H-N-H bond angle.

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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. The shape of a molecule is determined by the repulsion between electron pairs surrounding the central atom, which influences bond angles. For ammonia (NH3), the geometry is trigonal pyramidal, leading to a specific H-N-H bond angle that can be compared to other species like the amide ion and ammonium ion.

Hybridization

Hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals that can accommodate bonding. In ammonia (NH3), the nitrogen atom undergoes sp3 hybridization, resulting in a bond angle of approximately 107 degrees. In contrast, the ammonium ion (NH4+) has sp3 hybridization as well, but the bond angles are closer to 109.5 degrees due to the absence of lone pairs, while the amide ion (NH2-) has a different hybridization affecting its bond angles.

Lone Pairs and Bond Angles

Lone pairs of electrons can significantly affect bond angles in a molecule. They occupy more space than bonding pairs, causing bond angles to decrease. In the case of ammonia, the presence of one lone pair results in a smaller H-N-H bond angle compared to the ammonium ion, which has no lone pairs, leading to larger bond angles. The amide ion, with two lone pairs, will have the smallest bond angle due to increased repulsion.

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Consider the following XF₄ ions: PF₄^-, BrF₄^-, ClF₄⁺, and AlF₄^-. (a) Which of the ions have more than an octet of electrons around the central atom?

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Consider the following XF4 ions: PF4-, BrF4-, ClF4+, and AlF4-. (c) Which of the ions will have an octahedral electron-domain geometry?

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Textbook Question

Give the approximate values for the indicated bond angles in the following molecules: (d)

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Give the approximate values for the indicated bond angles in the following molecules: (c)

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The figure that follows contains ball-and-stick drawings of three possible shapes of an AF4 molecule. (c) Which of the following elements will lead to an AF4 molecule with the shape in (iii): Be, C, S, Se, Si, Xe? i.

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iii.

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