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NCERT Class XII Chapter
Atoms
Questions With Solutions

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Question : 6
Total: 18
A hydrogen atom initially in the ground level absorbs a photon, which excites it to the n = 4 level. Determine the wavelength and frequency of photon.
Solution:  
Energy of an electron in the nth orbit of H-atom, En = -
13.6
n2
 eV
Energy in the ground (n = 1) level, E1 = -
13.6
12
 = - 13.6 eV
Energy in the fourth (n = 4) level, E4 = -
13.6
42
 = - 0.85 eV
Energy radiated during emission
ΔE = E4–E1 = –0.85 – (–13.6) = 12.75 eV
As hυ = ΔE
∴ Frequency, υ =
ΔE
h
 =
12.75×1.6×10−19
6.6×10−34
 = 3.078 × 1015 Hz
Wavelength, λ =
c
υ
 =
3×108
3.078×1015
 = 0.9744 × 10−7 m = 974.4 Å
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