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The Solid State

Question
CBSEENCH12011337

Percentage of free space in a body centred cubic unit cell is 

  • 30%

  • 32%

  • 34%

  • 28%

Solution

B.

32%

Packing fraction

space equals space fraction numerator Volume space occupied space by space atoms space in space straight a space unit space cell over denominator Volume space of space the space unit space cell end fraction

For space body space centred space cube comma space
Packing space fraction space equals space fraction numerator 2 space straight x begin display style 4 over 3 end style πr cubed over denominator open parentheses begin display style fraction numerator 4 straight r over denominator square root of 3 end fraction end style close parentheses squared end fraction space equals space fraction numerator square root of 3 space straight pi over denominator 8 end fraction space equals space 0.68
where comma space edge space length space straight a space equals space fraction numerator 4 straight r over denominator square root of 3 end fraction
therefore comma space volume space space ccupiedf space equals space 68
and space volume space vacant space space equals space 32 percent sign