---
title: "In a study of crystal structures, a chemist compares pure crystalline \\(\\text{NaCl(s)}\\), in which each \\(\\text{Na}^+\\) cation is surrounded octahedrally by \\(6\\) \\(\\text{Cl}^-\\) anions, to pure crystalline \\(\\text{CsCl(s)}\\), where \\(\\text{Cs}^+\\) replaces \\(\\text{Na}^+\\). The ionic radius of \\(\\text{Cs}^+\\) (\\(167\\text{ pm}\\)) is significantly larger than that of \\(\\text{Na}^+\\) (\\(102\\text{ pm}\\)). Which of the following best predicts and explains the effect of substituting \\(\\text{Cs}^+\\) for \\(\\text{Na}^+\\) on both the coordination number of the cation and the magnitude of the lattice energy of the crystal?"
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date_modified: "2026-09-28T11:59:52+00:00"
---

# In a study of crystal structures, a chemist compares pure crystalline \(\text{NaCl(s)}\), in which each \(\text{Na}^+\) cation is surrounded octahedrally by \(6\) \(\text{Cl}^-\) anions, to pure crystalline \(\text{CsCl(s)}\), where \(\text{Cs}^+\) replaces \(\text{Na}^+\). The ionic radius of \(\text{Cs}^+\) (\(167\text{ pm}\)) is significantly larger than that of \(\text{Na}^+\) (\(102\text{ pm}\)). Which of the following best predicts and explains the effect of substituting \(\text{Cs}^+\) for \(\text{Na}^+\) on both the coordination number of the cation and the magnitude of the lattice energy of the crystal?

In a study of crystal structures, a chemist compares pure crystalline \(\text{NaCl(s)}\), in which each \(\text{Na}^+\) cation is surrounded octahedrally by \(6\) \(\text{Cl}^-\) anions, to pure crystalline \(\text{CsCl(s)}\), where \(\text{Cs}^+\) replaces \(\text{Na}^+\). The ionic radius of \(\text{Cs}^+\) (\(167\text{ pm}\)) is significantly larger than that of \(\text{Na}^+\) (\(102\text{ pm}\)). Which of the following best predicts and explains the effect of substituting \(\text{Cs}^+\) for \(\text{Na}^+\) on both the coordination number of the cation and the magnitude of the lattice energy of the crystal?

- **A.** The coordination number decreases because the larger \(\text{Cs}^+\) ion forces neighboring \(\text{Cl}^-\) ions farther apart, and the magnitude of the lattice energy increases because the larger electron cloud increases polarizability.
- **B.** The coordination number increases because the larger \(\text{Cs}^+\) ion accommodates more surrounding \(\text{Cl}^-\) ions, and the magnitude of the lattice energy increases because forming eight ionic interactions releases more energy than forming six.
- **C.** The coordination number increases because the larger \(\text{Cs}^+\) ion allows more \(\text{Cl}^-\) ions to pack around it without mutual repulsion, and the magnitude of the lattice energy decreases because the greater interionic distance weakens Coulombic attractions.
- **D.** The coordination number remains unchanged because the \(1:1\) stoichiometry dictates an octahedral geometry, and the magnitude of the lattice energy decreases because the lower ionization energy of \(\text{Cs}\) reduces electrostatic attraction.

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