NEET Chemistry NEET PYQ-The Solid State

  • question_answer
    Percentage of free space in a body centred cubic unit cell is                   [AIPMT (S) 2008]

    A) 30%

    B) 32%     

    C) 34%

    D) 28%

    Correct Answer: B

    Solution :

    [b] Key Idea: Packing fraction
    \[=\frac{\text{Volume occupied by atoms in a unit cell}}{\text{Volume of the unit cell}}\]
    For body centred cube,
    Packing fraction \[=\frac{2\times \frac{4}{3}\pi {{r}^{3}}}{{{\left( \frac{4r}{\sqrt{3}} \right)}^{3}}}=\frac{\sqrt{3}\pi }{8}=0.68\]             where, edge length \[a=\frac{4r}{\sqrt{3}}\]
    \[\therefore \]Volume occupied = 68% and volume vacant = 32%


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