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Ch.8 - Basic Concepts of Chemical Bonding
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.
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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.
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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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