Atomic Spectra

A spectrum is an assembly of energy levels in the form of radiations emitted by an atom in its excited state. Every atom gives discontinuous line spectra. Each line in the spectra corresponds to a specific wavelength and it is unique to a given element. No two elements give same pattern of lines in their spectra.

Atomic emission spectra

When a substance is heated to a high temperature, the atoms in the vapours get energized. These energized atoms then return to the ground state by emitting electromagnetic radiations of certain definite wavelength. A series of bright lines separated from each other by dark spaces is obtained and this is called atomic emission spectra.

Fig: 3.9 - Mechanism of emission spectra

Atomic absorption spectra

When the atomic vapours from a sample are placed in the path of white light from an arc lamp, it absorbs the light of certain characteristic wavelengths and the light of other wavelengths get transmitted. In such conditions a series of dark lines on a white background are formed. This is called an absorption spectrum.

The dark lines in the absorption spectrum and the bright lines in the emission spectrum of a given element appear at the same wavelength.

Fig: 3.10 - Comparison of Absorption and emission spectra of sodium vapour

Since each element gives a definite pattern of lines at certain definite frequencies or wavelengths, the atomic spectra is used in chemical analysis to identify and estimate the elements present in any sample.

Atomic spectra of hydrogen atom

Hydrogen is the simplest element with its atom having only one electron. Hence, the atomic spectrum of hydrogen has played a significant role in the development of atomic structure. In the emission spectrum of hydrogen, when an electric discharge is passed through hydrogen gas, the molecules of hydrogen break into atoms. The hydrogen atoms get energized and go into an excited state. The excited atoms then return to the ground state by emitting light. Hydrogen atoms emit bluish light. On passing this light through a prism, a discontinuous line spectrum consisting of several sharp lines is obtained. This is the line spectrum of hydrogen.

Four sharp coloured lines were observed in the visible region of this spectrum by Balmer, in the ultra violet region by Lyman, in the infrared region by Paschen, Brackett and Pfund. These series of lines are named after these scientists who discovered them. Balmer expressed these lines in terms of inverse of their wavelength () by a mathematical relation, which was later modified by Rydberg.

where 'R_H' is the Rydberg's constant and 'n₁', 'n₂' are integers with values equal to or greater than 3 and 'l' is the wavelength.

Fig: 3.11 - Line spectrum of hydrogen atom

Problem

6. Calculate the wavelength of the spectral line when the electron in the hydrogen atom undergoes a transition from 4^th energy level to 2^nd energy level. What is the colour of the radiation?

Solution

According to Rydberg's equation

here, n₁ = 2, n₂ = 4 and R = 109678 cm^-1

l = 486 x 10^-9 m = 486 nm.