The Solid State

The Solid State

Question

The edge length of a face centred cubic cell of an ionic substance is 508 pm. If the radius of the cation is 110 pm, the radius of the anion is 

  • 144 pm

  • 288 pm

  • 398 pm

  • 618 pm

Answer

A.

144 pm

For FCC lattice (assuming cation in octahedral void and anion in FCC)
a = 508 pm
(r+ + r-) = a/2 = 508/2 = 254 pm
(r+ + r-) = 254 pm
r- = 254-110 = 144 pm

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