2. Atomic Spectra

Atomic Spectra

• Each element emits a characteristic spectrum of radiation.
• In the excited state, the atoms emit radiations of a spectrum, which contains certain specific wavelengths only. This spectrum is termed as emission line spectrum and it consists of bright lines on a dark background.
• The spectrum emitted by atomic hydrogen is shown in the figure below.

• Spectral Series

When the electron in a hydrogen atom jumps from higher energy level to the lower energy level, the difference of energies of the two energy levels is emitted as a radiation of particular wavelength. It is called a spectral line.

In H-atom, when an electron jumps from the orbit n_i to orbit n_f, the wavelength of the emitted radiation is given by,

Where,

R → Rydberg’s constant = 1.09678 ×10⁷ m⁻¹

For transition of the electron between two different energy levels, the spectral lines of different wavelengths are obtained. These spectral lines are found to fall into a number of spectral series as discussed below.

1. Lyman series

For Lyman series, n_f = 1 and n_i = 2, 3, 4, …

These spectral lines lie in ultraviolet region.

2. Balmer series

For Balmer series, n_f = 2 and n_i = 3, 4, 5, …

Where, n_i = 3, 4, 5, …

These spectral lines lie in the visible region.

3. Paschan series

For Paschan series, n_f = 3 and n_i = 4, 5, 6, …

These spectral lines lie in the IR − region.

4. Brackett series

For Brackett series, n_f = 4 and n_i = 5, 6, 7, …

The spectral lines of this series lie in IR − region.

5. Pfund series

For Pfund series, n_f = 5 and n_i = 6, 7, 8, …

These series lie in the far infrared region.