Ch.8 - Basic Concepts of Chemical Bonding

Brown - Chemistry: The Central Science 14th Edition

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Brown 14th Edition

Ch.8 - Basic Concepts of Chemical Bonding

Problem 88c

Chapter 8, Problem 88c

A major challenge in implementing the 'hydrogen economy' is finding a safe, lightweight, and compact way of storing hydrogen for use as a fuel. The hydrides of light metals are attractive for hydrogen storage because they can store a high weight percentage of hydrogen in a small volume. For example, NaAlH₄ can release 5.6% of its mass as H2 upon decomposing to NaH(s), Al(s), and H₂(g). NaAlH₄ possesses both covalent bonds, which hold polyatomic anions together, and ionic bonds. (c) Based on electronegativity differences, predict the identity of the polyatomic anion. Draw a Lewis structure for this ion.

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Identify the elements present in NaAlH_4: sodium (Na), aluminum (Al), and hydrogen (H).

Consider the electronegativity values: Na (0.93), Al (1.61), and H (2.20).

Determine the most electronegative element: hydrogen (H) is more electronegative than both Na and Al.

Predict the polyatomic anion: AlH_4^- is likely, as Al is less electronegative than H, allowing H to form a polyatomic anion with Al.

Draw the Lewis structure for AlH_4^-: Place Al in the center, surround it with four H atoms, and ensure the total charge is -1 by adding an extra electron to complete the octet around Al.

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Key Concepts

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

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons in a chemical bond. It plays a crucial role in determining the type of bond formed between atoms, whether ionic or covalent. In the context of NaAlH4, understanding the electronegativity differences between sodium, aluminum, and hydrogen helps predict the nature of the bonds and the resulting polyatomic anion.

Ionic and Covalent Bonds

Ionic bonds form between atoms with significantly different electronegativities, resulting in the transfer of electrons from one atom to another, creating charged ions. Covalent bonds, on the other hand, involve the sharing of electrons between atoms with similar electronegativities. NaAlH4 contains both types of bonds, which is essential for understanding its structure and the behavior of the polyatomic anion formed during hydrogen release.

Lewis Structures

Lewis structures are diagrams that represent the bonding between atoms in a molecule and the lone pairs of electrons that may exist. They are useful for visualizing the arrangement of electrons and predicting the geometry of molecules. Drawing the Lewis structure for the polyatomic anion in NaAlH4 will help clarify its electron distribution and the nature of its bonds, aiding in the understanding of its stability and reactivity.

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Related Practice

Textbook Question

Calculate the formal charge on the indicated atom in each of the following molecules or ions: (a) the central oxygen atom in O₃ (b) phosphorus in PF₆^- (c) nitrogen in NO₂ (d) iodine in ICl₃ (e) chlorine in HClO₄ (hydrogen is bonded to O).

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Textbook Question

Although I₃^- is a known ion, F₃^- is not. (b) One of your classmates says that F₃^- does not exist because F is too electronegative to make bonds with another atom. Give an example that proves your classmate is wrong.

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Textbook Question

The substance chlorine monoxide, ClO(g), is important in atmospheric processes that lead to depletion of the ozone layer. The ClO molecule has an experimental dipole moment of 1.24 D, and the Cl—O bond length is 160 pm. (c) Using formal charges as a guide, propose the dominant Lewis structure for the molecule. (g), is important in atmospheric processes that lead to depletion of the ozone layer. The ClO molecule has an experimental dipole moment of 1.24 D, and the Cl—O bond length is 160 pm. (d) The anion ClO exists. What is the formal charge on the Cl for the best Lewis structure for ClO^-?

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Textbook Question

The hypochlorite ion, ClO^-, is the active ingredient in bleach. The perchlorate ion, ClO₄^-, is a main component of rocket propellants. Draw Lewis structures for both ions. (b) What is the formal charge of Cl in the perchlorate ion, assuming the Cl—O bonds are all single bonds?

921

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Textbook Question

(c) The measured dipole moment of BrCl is 0.57 D. If you assume the bond length in BrCl is the sum of the atomic radii, what are the partial charges on the atoms in BrCl using the experimental dipole moment?

(b) Using these partial charges and the atomic radii given in Figure 7.8, estimate the dipole moment of the molecule.

881

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