When an electron in a hydrogen atom returns to the ground state from $n = 5$, the maximum number of distinct spectral lines that can be emitted is
A$4$
B$10$
C$5$
D$15$
Answer & Solution
Correct answer: B. $10$
Number of distinct transitions from level $n$ is $\binom{n}{2} = n(n-1)/2$. For $n=5$: $5 \cdot 4/2 = 10$.
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