Chemistry · Structure Of Atom · NEET
Each series is named by the FINAL level the electron lands on. Paschen series ends at n=3. Brackett series ends at n=4. Pfund series ends at n=5. The electron starts from any higher level and drops down to this fixed lower level. For example, in the Paschen series the electron can come from n=4, 5, 6... but it always ends at n=3.
Yes. Paschen, Brackett and Pfund series all lie in the infrared (IR) region of the electromagnetic spectrum. This is because the energy gaps for these jumps are small, so the light has a long wavelength (low energy). Only the Balmer series (ends at n=2) is in the visible region, and only the Lyman series (ends at n=1) is in the ultraviolet region.
The five series match the ending levels n=1, 2, 3, 4, 5 in order. So: Lyman = n=1 (UV), Balmer = n=2 (visible), Paschen = n=3 (IR), Brackett = n=4 (IR), Pfund = n=5 (IR). A simple sentence: 'Little Boys Prefer Beautiful Presents' for Lyman, Balmer, Paschen, Brackett, Pfund. The last three are all IR.
Use the Rydberg formula: 1/λ = R (1/n1² − 1/n2²), where R = 109677 cm⁻¹ is the Rydberg constant, n1 is the lower (ending) level, and n2 is the higher (starting) level. For Paschen n1=3, for Brackett n1=4, for Pfund n1=5. n2 is always greater than n1. This is why NEET asks you to know the ending level of each series.
Balmer series ends at n=2, and its lines fall in the VISIBLE region (the colours you can see). Paschen, Brackett and Pfund all end at n=3 or higher, so their energy gaps are smaller and their light is infrared, which the eye cannot see. NEET often mixes these up to test if you know that only Balmer is visible.
Which of the following series of transitions in the spectrum of the hydrogen atom falls in the visible region?
The ratio of the wavelengths of the light absorbed by a hydrogen atom when it undergoes n=2 → n=3 and n=4 → n=6 transitions, respectively, is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The Pfund series is the set of spectral lines of hydrogen where the electron falls from a higher level (n=6, 7, 8...) down to n=5. All Pfund lines lie in the far infrared region.
The Brackett series is in the infrared region. It ends at n=4, so the energy gaps are small and the light has a long, invisible (infrared) wavelength.
For the Paschen series, the lower level n1 = 3. The electron always ends at n=3, coming from n=4, 5, 6 and higher.
Only the Balmer series is visible to the eye. Lyman is ultraviolet, and Paschen, Brackett and Pfund are all infrared, so none of these three can be seen.
They can appear in both. In emission the electron drops down (releases light); in absorption it jumps up. The named series (Paschen etc.) usually refer to emission ending at the fixed lower level, but the same wavelengths appear in absorption too.
