Discussion
The greatest wavelength of the radiation that will ionize unexcited hydrogen atom is
Answer: C
The greatest wavelength is about 910 Å.
Ionising a ground-state hydrogen atom needs 13.6 eV. The longest wavelength that can supply it is
\[ \lambda = \frac{hc}{E} = \frac{12400}{13.6} \approx 912 \]
in Ångstrom units, using \(hc \approx 12400\) eV Å.
This is the series limit of the Lyman series. Longer wavelengths such as 1400 or 1820 Å carry too little energy; shorter ones such as 450 or 700 Å also ionise, but the question asks for the greatest.
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