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A beam of electrons having energy 12.5 eV is directed at hydrogen gas in the gaseous state at room temperature. The emitted spectrum will contain:
- 2 lines in the Lyman series and 1 line in the Balmer series
- 3 lines in the Lyman series
- 1 line in the Lyman series and 2 lines in the Balmer series
- 3 lines in the Balmer series
Correct answer: 2 lines in the Lyman series and 1 line in the Balmer series
Solution
From n=1, exciting to n=3 needs 12.09 eV (possible) but to n=4 needs 12.75 eV (>12.5 eV, not possible). Allowed transitions are 3->1, 2->1 (Lyman, 2 lines) and 3->2 (Balmer, 1 line): 2 Lyman + 1 Balmer.
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