alleryThe ratio of the radii of the third Bohr orbit of $He^+$ and the first Bohr orbit of $H$ is
A$3$
B$9$
C$6$
D$9/2$
Answer & Solution
Correct answer: D. $9/2$
$r_n = (0.529\,Å)\,n^2/Z$.
For $He^+$ ($Z=2$, $n=3$): $r = 9/2 \cdot 0.529\,Å$.
For $H$ ($Z=1$, $n=1$): $r = 0.529\,Å$. Ratio $= 9/2$.
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