Problem: 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.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?

Problem Details
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.
Period 2 Group 1A contains Li 1.28 and Li cation 0.90.  Period 3 Group 1A Na 1.66 and Na+ 1.16.  Period 3 Group 1A K 2.03 and K+ 1.52.  Period 4 Group 1A Rb 2.20 and Rb+ 1.66.Period 1 Group 2A contains Be 0.96 and Be2+ 0.59.  Period 2 Group 2A contains Mg 1.41 and Mg2+ 0.86. Period 3 Group 2A contains Ca 1.76 and Ca2+ 1.76.  Period 4 contains Sr 1.95 and Sr2+ 1.32. Period 1 Group 3A contains B 0.84 and B3+ 0.41.  Period 2 Group 3A contains Al 1.21 and Al3+ 0.68.  Period 3 contains Ga 1.22 and Ga3+ 0.76.  Period 4 contains In 1.42 and In3+ 0.94. Period 1 Group 6A contains O 0.66 and O2- 1.26. Period 2 Group 6A contains S 1.05 and S2- 1.70. Period 3 contains Se 1.20 and Se2- 1.84.  Period 4 Te 1.38 and Te2- 2.07. Period 1 Group 7A contains F 0.57 and F- 1.19.  Period 2 Group 7A contains Cl 1.02 and Cl- 1.67.  Period 3 contains Br 1.20 and Br- 1.82.  Period 4 I 1.39 and I- 2.06.
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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