Which of the following has the smallest size?

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(D) The size of hydrogen-like species is given by the formula $r \propto \frac{n^2}{Z}$,where $n$ is the principal quantum number and $Z$ is the atomi

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$Li^{2+}$

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(D) The size of hydrogen-like species is given by the formula $r \propto \frac{n^2}{Z}$,where $n$ is the principal quantum number and $Z$ is the atomic number.For $H$ $(n=1, Z=1)$,$r \propto \frac{1^2}{1} = 1$.For $He^{+}$ $(n=1, Z=2)$,$r \propto \frac{1^2}{2} = 0.5$.For $H^{-}$ ($n=1, Z=1$,but with two electrons,the effective nuclear charge decreases due to shielding,leading to a larger size than $H$).For $Li^{2+}$ $(n=1, Z=3)$,$r \propto \frac{1^2}{3} = 0.33$.Comparing the values,$Li^{2+}$ has the smallest radius.

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