EVIDENCE FOR THE QUANTIZED ELECTRONIC STATES-ATOMIC SPECTRA
When an element is excited by some method such as by heating, by passing electric current or by passing electric discharge, the atoms of the element emit electromagnetic radiations of definite frequencies. The arrangement of these radiations in the order of increasing wavelength or decreasing frequencies is called emission spectrum of the element. Since the radiations in the spectrum are emitted due to energy changes taking place in the atoms, this spectrum is also atomic spectrum.
The instrument used for obtaining a spectrum is called spectroscope or spectrograph. In the spectroscope light is passed through a prism and the emergent light is observed. It is a well-established fact that ray of light undergoes deviation when passed through a prism. The angle of deviation is directly proportional to the frequency of radiation. Thus, if we pass a beam of radiations having different frequencies through a prism, these radiations undergo unequal deviation and get arranged in order of decreasing frequencies. For example, a beam of ordinary light splits up into seven colours (VIBGYOR) after passing through the prism. This phenomenon of splitting of a beam of light into radiations of differentf requencies after passing through the pr.ism is called dispersion and the pattern of radiations obtained after dispersion of beam is called spectrum. In case of dispersion of sunlight, the seven colours obtained change from violet to red without any discontinuity, which means each colour blends into the other. Such a spectrum is called continuous spectra.
In continuous spectrum, the radiations corresponding to all the wavelengths (within a certain range) are present.
On the other hand, atomic spectra of most of the elements consist of a number of coloured lines separated by dark bands. That is why atomic spectrum is also known as line spectrum. The various lines in the line spectrum correspond to the radiations of different wavelengths, emitted by the excited element. The lines in the line spectrum of an element are characteristic of the atoms of the element. Therefore, atomic spectrum of an element can be used to identify the element and is sometimes called fingerprint of its atoms.
Elements like rubidium (Rb), caesium (Cs), indium (In), scandium (Sc), etc., were discovered when their minerals were analysed by spectroscopic methods. The element helium was discovered in the sun by study of spectrum of sunlight.
When a beam of continuous light is passed through a tube containing vapours or solution of the substance and the transmitted light is analysed with the help of a spectrometer, it is observed that the spectrum obtained contains a number of dark lines in otherwise continuous spectrum (Fig. 5.16). These dark lines appear due to the absorption of radiations of corresponding wavelengths by the substance. The dark lines in the absorption spectrum of a substance appear at the same position as the bright lines in the emission spectrum of the substance. For example, emission spectrum of sodium has two bright lines in the yellow region of 589.0 nm and 589.6 nm whereas its absorption spectrum has two dark lines corresponding to the same wavelengths. Generally more lines are observed in the emission spectrum than in its absorption spectrum. “Therefore, absorption spectrum is taken if simplification of the spectrum is desirable.
Fig. 5.16. Absorption spectrum
The study of emission or absorption spectra is called spectroscopy.
The atomic spectrum of hydrogen has proved quite helpful in the development of atomic structure. It can be obtained by passing the light being emitted from the discharge tube containing hydrogen at low pressure, through the spectrograph as shown in Fig. 5.17.
Fig. 5.17 Emission spectrum of hydrogen
When an electric discharge is passed through hydrogen gas its molecules dissociate into hydrogen atoms. The excited hydrogen atoms, thus produced, emit electromagnetic radiations of discrete frequencies. The spectrum obtained consists of a large number of sharp lines. Each line corresponds to a particular frequency of light emitted by hydrogen atoms. Lines in the emission spectrum of hydrogen are present in ultraviolet, visible and infrared regions. While the lines in the ultraviolet and visible regions can be directly obtained on photographic plate, the lines in the infrared region are detected by other methods.
The lines in the emission spectrum of hydrogen are classified into five series as follows:
(i) Lyman series …… Ultraviolet region
(ii) Balmer series …… Visible region
(iii) Paschen series …… Infrared region
(iv) Brackett series …… Infrared region
(v) Pfund series· . ….. Infrared region
The complete spectrum of hydrogen is shown in Fig. 5.17a.