Question

Sol. Key concept: A hydrogen spectral series is identified by its final principal quantum number.

$$\frac{1}{\lambda} = R(\frac{1}{{n_{1}}^{2}} - \frac{1}{{n_{2}}^{2}}),\ \ \ \ \ n_{2} > n_{1}$$

(a) Correct: Balmer transitions terminate at n₁ = 2 and originate from n₂ = 3, 4, 5, …, so n₂ > 2.

(b) False: Pfund transitions terminate at n₁ = 5, so their first reciprocal-square term is 1/5², not 1/3². The latter belongs to the Paschen series.

(c) False: the Humphreys series has the fixed final level n₁ = 6; it is n₂ that must be greater than 6.

(d) Correct: Paschen transitions end at n₁ = 3 and lie in the infrared region. Hence (a) and (d).