CET Karnataka Engineering CET - Karnataka Engineering Solved Paper-2004

  • question_answer
    Which state of triply ionised Beryllium  \[(B{{e}^{+++}})\] has the same orbital radius as that of the ground state of hydrogen?

    A)  \[n=3\]                               

    B)  \[n=4\]

    C)  \[n=1\]                               

    D)  \[n=2\]

    Correct Answer: D

    Solution :

    Radius of orbit of electron in nth excited state of hydrogen                 \[r=\frac{{{\varepsilon }_{0}}{{h}^{2}}{{n}^{2}}}{\pi \,mZ{{e}^{2}}}\] \[\therefore \]  \[r\propto \frac{{{n}^{2}}}{Z}\]                                                  ... (1) \[\therefore \]  \[\frac{{{r}_{1}}}{{{r}_{2}}}=\frac{n_{1}^{2}}{n_{2}^{2}}\times \frac{{{Z}_{2}}}{{{Z}_{1}}}\]                 But         \[{{r}_{1}}={{r}_{2}}\] So,          \[n_{2}^{2}=n_{1}^{2}\times \frac{{{Z}_{2}}}{{{Z}_{1}}}\] Here: \[{{n}_{1}}=1\] (ground state of hydrogen), \[{{Z}_{1}}=1\] (atomic number of hydrogen), \[{{Z}_{2}}=4\] (atomic number of Beryllium)                 \[\therefore \]  \[n_{2}^{2}={{(1)}^{2}}=\times \frac{4}{1}\] or            \[n_{2}^{2}=4\] or            \[{{n}_{2}}=2\]


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