Ch.8 - Basic Concepts of Chemical Bonding

Problem 82a

Chapter 8, Problem 82a

The ionic compound CaO crystallizes with the same structure as sodium chloride (Figure 8.3). (a) In this structure, how many O2- are in contact with each Ca2+ ion (Hint: Remember the pattern of ions shown in Figure 8.3 repeats over and over again in all three directions.)

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Identify the crystal structure of sodium chloride (NaCl), which is a face-centered cubic (FCC) lattice.

In the NaCl structure, each sodium ion (Na+) is surrounded by six chloride ions (Cl-), forming an octahedral coordination.

Since CaO crystallizes in the same structure as NaCl, each calcium ion (Ca2+) will also be surrounded by six oxide ions (O2-).

Visualize the repeating pattern in three dimensions, where each Ca2+ ion is at the center of an octahedron formed by O2- ions.

Conclude that in the NaCl-type structure, each Ca2+ ion is in contact with six O2- ions.

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Key Concepts

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

Ionic Compounds and Crystal Lattices

Ionic compounds, like CaO, consist of positively and negatively charged ions held together by electrostatic forces. They form a crystal lattice structure, where ions are arranged in a repeating pattern that maximizes attraction and minimizes repulsion. Understanding this arrangement is crucial for determining how many ions of one type are in contact with another in the lattice.

Coordination Number

The coordination number refers to the number of nearest neighbor ions surrounding a central ion in a crystal lattice. In the case of CaO, the coordination number of Ca2+ indicates how many O2- ions are directly adjacent to each calcium ion. This concept is essential for analyzing the spatial arrangement of ions in the crystal structure.

Sodium Chloride Structure

The sodium chloride (NaCl) structure is a common example of a face-centered cubic lattice where each Na+ ion is surrounded by six Cl- ions and vice versa. This arrangement serves as a model for understanding the structure of other ionic compounds, such as CaO, which adopts a similar lattice. Recognizing this similarity helps in visualizing the ion interactions and determining the number of surrounding ions.

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Textbook Question

Consider the lattice energies of the following Group 2A compounds: BeH₂, 3205 kJ/mol; MgH₂, 2791 kJ/mol; CaH₂, 2410 kJ/mol; SrH₂, 2250 kJ/mol; BaH₂, 2121 kJ/mol. (c) Consider BeH₂. Does it require 3205 kJ of energy to break one mole of the solid into its ions, or does breaking up one mole of solid into its ions release 3205 kJ of energy?

Textbook Question

Consider the lattice energies of the following Group 2A compounds: BeH₂, 3205 kJ/mol; MgH₂, 2791 kJ/mol; CaH₂, 2410 kJ/mol; SrH₂, 2250 kJ/mol; BaH₂, 2121 kJ/mol. (d) The lattice energy of ZnH₂ is 2870 kJ/mol. Considering the trend in lattice enthalpies in the Group 2 compounds, predict which Group 2 element is most similar in ionic radius to the Zn²⁺ ion.

Open Question

A metal oxide has a lattice energy in the range of 3300 kJ/mol. Is the charge on the metal ion M likely to be 1+, 2+, or 3+? Explain.

Textbook Question

Construct a Born–Haber cycle for the formation of the hypothetical compound NaCl₂, where the sodium ion has a 2⁺ charge (the second ionization energy for sodium is given in Table 7.2). (a) How large would the lattice energy need to be for the formation of NaCl₂ to be exothermic?

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Open Question

A classmate of yours is convinced that he knows everything about electronegativity. (a) In the case of atoms X and Y having different electronegativities, he says, the diatomic molecule X–Y must be polar. Is your classmate correct? (b) Your classmate says that the farther the two atoms are apart in a bond, the larger the dipole moment will be. Is your classmate correct?

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Consider the collection of nonmetallic elements O, P, Te, I, and B. (a) Which two would form the most polar single bond?

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