We are asked to determine the radius of an iridium atom, given that Ir has an FCC crystal structure.
For FCC, n = 4 atoms/unit cell, and VC = 16R3 2 [Equation (3.4)]. Now,
ρ =
nAIr
V
CNA
And solving for R from the above two expressions yields
R =
nAIr
16ρNA 2
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1/3
= ( ) 4 atoms/unit cell ( ) 192.2 g/mol
( )2 ( ) 16 22.4 g/cm3 ( )6.023 x 1023 ( ) atoms/mol
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1/3
= 1.36 x 10-8 cm = 0.136 nm