Answer
The electron goes from n = 5 to n = 3.
Work Step by Step
The longer the wavelength, the smaller the energy. For the Paschen series, the smallest energy and longest wavelength is when the electron goes from n = 4 to n = 3.
Therefore the second smallest energy and second-longest wavelength is when the electron goes from n = 5 to n = 3.
Using Figure 27-29, we confirm the answer: The second longest wavelength corresponds to the transition that has the second smallest energy.
