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JEE Main levelSingle correct optionDifficulty 2 of 3ss025

According to the radius-ratio rule (limiting ratios 0.225, 0.414, 0.732), which ion pair is expected to have coordination number 8 for the cation? Radii (pm):

A $Na X^{+}$ (95) and $Cl X^{-}$ (181)
B $Mg X^{2 +}$ (72) and $O X^{2 -}$ (140)
C $Zn X^{2 +}$ (74) and $S X^{2 -}$ (184)
D $Cs X^{+}$ (169) and $Cl X^{-}$ (181)

Answer: D

Compute

r_{+} / r_{-}

: Cs/Cl

= 169/181 = 0.934

(above 0.732, CN 8); Na/Cl

= 0.525

(CN 6); Zn/S

= 0.402

(CN 4); Mg/O

= 72/140 = 0.514

(CN 6). Only the first pair gives CN 8.

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