Ch.7 - Periodic Properties of the Elements

Problem 36b

Chapter 7, Problem 36b

In the ionic compounds LiF, NaCl, KBr, and RbI, the measured cation–anion distances are 201 pm (Li–F), 282 pm (Na–Cl), 330 pm (K–Br), and 367 pm (Rb–I), respectively. (b) Calculate the difference between the experimentally measured ion–ion distances and the ones predicted from Figure 7.8.

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Identify the ionic radii of the cations and anions from the provided table: Li+ (90 pm), Na+ (116 pm), K+ (152 pm), Rb+ (166 pm), F- (119 pm), Cl- (167 pm), Br- (182 pm), I- (206 pm).

Calculate the predicted ion–ion distances by adding the radii of the respective cations and anions: LiF (Li+ + F-), NaCl (Na+ + Cl-), KBr (K+ + Br-), RbI (Rb+ + I-).

Compare the experimentally measured ion–ion distances with the predicted distances: LiF (201 pm), NaCl (282 pm), KBr (330 pm), RbI (367 pm).

Determine the difference between the experimentally measured distances and the predicted distances for each compound.

Summarize the differences for each compound: LiF, NaCl, KBr, and RbI.

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Key Concepts

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

Ionic Radii

Ionic radii refer to the effective size of an ion in a crystal lattice, which is influenced by its charge and the number of electrons. Cations, which are positively charged, are generally smaller than their neutral atoms due to the loss of electrons, while anions, negatively charged, are larger due to the addition of electrons. Understanding ionic radii is crucial for predicting the distances between ions in ionic compounds.

Cation-Anion Distances

Cation-anion distances are the measured distances between the centers of cations and anions in an ionic compound. These distances can be experimentally determined and are influenced by the sizes of the ions involved. Analyzing these distances helps in understanding the stability and structure of ionic compounds, as well as their physical properties.

Experimental vs. Predicted Values

In chemistry, comparing experimental values with predicted values allows for the validation of theoretical models. In this context, the predicted ion-ion distances can be derived from the ionic radii, while the experimental values are obtained through measurement. Discrepancies between these values can provide insights into the nature of ionic bonding and the arrangement of ions in the crystal lattice.

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Textbook Question

In the ionic compounds LiF, NaCl, KBr, and RbI, the measured cation–anion distances are 201 pm (Li–F), 282 pm (Na–Cl), 330 pm (K–Br), and 367 pm (Rb–I), respectively. (c) What estimates of the cation– anion distance would you obtain for these four compounds using neutral atom bonding atomic radii? Are these estimates as accurate as the estimates using ionic radii?

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