In the ionic compounds LiF, NaCl, KBr, and RbI, the measured cation–anion distan
ID: 535899 • Letter: I
Question
In the ionic compounds LiF, NaCl, KBr, and RbI, the measured cation–anion distances are 2.01 Å (Li–F), 2.82 Å (Na-Cl), 3.30 Å (K–Br), and 3.67 Å (Rb–I), respectively. Calculate the difference between the experimentally measured ion-ion distances provided at the beginning of this assignment and the ones you have predicted in problem 1. You must show how you calculated the difference for each molecule by providing a step-by-step solution for each molecule to get full credit (6 points for each molecule; total possible points for this section is 24 points). Assuming that we have an accuracy of 0.04Å in the measurement, please indicate for each molecule if your predicted ion-ion distances are accurate or not accurate with respect to their experimental ion-ion distances (2 points for each molecule; total possible points for this section is 12 points). A total of 32 total points is possible for this question.
Explanation / Answer
The measured cation–anion distances are 2.01 Å (Li–F), 2.82 Å, (Na-Cl), 3.30 Å (K–Br), and 3.67 Å (Rb–I), respectively
LiF
The ionic radius of cation Li^+ = 0.90Å
The ionic radius of anion F^- = 1.19Å
The cation–anion distances = the ionic radius of Li^+ + the ionic radius of anion F^-
= 0.90Å + 1.19Å = 2.09Å
NaCl
The ionic radius of cation Na^ + = 1.16 Å
The ionic radius of anion Cl^-= 1.67 Å
The cation–anion distances = the ionic radius of Na^+ + the ionic radius of anion Cl^-
= 1.16Å + 1.67Å =2.83Å
KBr
The ionic radius of cation K^+ = 1.52Å
The ionic radius of anion Br^-= 1.82Å
The cation–anion distances = the ionic radius of K^+ + the ionic radius of anion Br^-
= 1.52Å + 1.82Å = 3.34 Å
RbI
The ionic radius of cation Rb^+ = 1.66Å
The ionic radius of anion I^-= 2.06Å
The cation–anion distances = the ionic radius of Rb^+ + the ionic radius of anion I^-
= 1.66Å + 2.06Å = 3.72 Å
The difference between the experimentally measured ion-ion distances and ion-ion distances given in text book
LiF = 2.09Å - 2.01 Å = 0.08 Å
NaCl = 2.83Å - 2.82Å = 0.01Å
KBr = 3.34 Å -3.30 Å = 0.04 Å
RbI = 3.72 Å -3.67 Å = 0.05 Å
Assuming that we have an accuracy of 0.04Å in the measurement, our predicted ion-ion distances for NaCl and KBr are accurate. But our predicted ion-ion distances for LiF and RbI are not accurate, because difference is greater than 0.04 Å.
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