JEE Main & Advanced JEE Main Paper (Held on 08-4-2019 Afternoon)

  • question_answer
    Consider the bcc unit cells of the solids 1 and 2 with the position of atoms as shown below. The radius of atom B is twice that of atom A. The unit cell edge length is 50% more in solid 2 than in 1. What is the approximate packing efficiency in solid 2?             [JEE Main 8-4-2019 Afternoon]

    A) 45%                

    B) 65%

    C) 90%    

    D)   75%

    Correct Answer: C

    Solution :

    \[p.f.=\frac{{{\left( {{z}_{eff}}\times \frac{4}{3}\pi r_{A}^{3} \right)}_{A}}+{{\left( {{z}_{eff}}\times \frac{4}{3}\pi r_{B}^{3} \right)}_{B}}}{{{a}^{3}}}\]           \[2({{r}_{A}}+{{r}_{B}})=\sqrt{3}a\]           \[\Rightarrow \]\[2({{r}_{A}}+2{{r}_{A}})=\sqrt{3}a\]\[\Rightarrow \]\[2\sqrt{3}{{r}_{A}}=a\]        \[\Rightarrow \]\[p.f.=\frac{1\times \frac{4}{3}\pi r_{A}^{3}+\frac{4}{3}\pi \left( 8r_{A}^{3} \right)}{8\times 3\sqrt{3}\,\,{{r}_{A}}^{3}}=\frac{9\times \frac{4}{3}\pi }{8\times 3\sqrt{3}}=\frac{\pi }{2\sqrt{3}}\] p. efficiency\[=\frac{\pi }{2\sqrt{3}}\times 100\simeq 90%\]


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