Draw a neat, labelled energy level diagram for H atom showing the transitions. Explain the series of spectral lines for H atom, whose fixed inner orbit numbers are 3 and 4 respectively.

#### Solution

**Paschen series:**

i. The spectral lines of this series correspond to the transition of an electron from some higher energy state to 3rd orbit.

ii. For paschen series, p = 3 and n = 4, 5,...

The wave numbers and the wavelengths of the spectral lines constituting the Paschen series are given by,

`barv=1/lambda=R(1/3^2-1/n^2)`

iii. Paschen series lies in the infrared region of the spectrum which is invisible and contains infinite number of lines.

iv. Wavelengths for n = 4 and 5 are 18750 Å and 12820 Å respectively.

**Brackett series:**

i. The spectral lines of this series corresponds to the transition of an electron from a higher energy state to the 4th orbit.

ii. For this series, p = 4 and n = 5, 6, 7,...

The wave numbers and the wavelengths of the spectral lines constituting the Brackett series are given by,

`barv=1/lambda=R(1/4^2-1/n^2)`

iii. This series lies in the ne ar infrared region of the spectrum and contains infinite number of lines. Wavelengths for n = 5 and 6, are 40518 Å and 26253 Å respectively