Q. If radius of second Bohr orbit of the $He^+$ ion is 105.8 pm, what is the radius of third Bohr orbit of $Li^{2+}$ ion?
A) 158.7 Å
B) 158.7 pm
C) 15.87 pm
D) 1.587 pm
Explanation
[Solution] $r \propto n^2/Z$. For $He^+$ (n = 2, Z = 2): $r = 2a_0 = 105.8$ pm, so $a_0 = 52.9$ pm. For $Li^{2+}$ (n = 3, Z = 3): $r = 3a_0 = 158.7$ pm. Answer (2).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.