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44 chemistry 2.3.3 E vidence for the quantized* spectrum. A continuum of radiation is passed E lectronic Energy Levels: Atomic through a sample which absorbs radiation of spectra certain wavelengths. The missing wavelength The speed of light depends upon the nature which corresponds to the radiation absorbed by the matter, leave dark spaces in the bright of the medium through which it passes. As a result, the beam of light is deviated or refractedontinuous spectrum. from its original path as it passes from one The study of emission or absorption medium to another. It is observed that when spectra is referred to as spectroscopy. The a ray of white light is passed through a prism, spectrum of the visible light, as discussed the wave with shorter wavelength bends more above, was continuous as all wavelengths (red to violet) of the visible light are represented in than the one with a longer wavelength. Since ordinary white light consists of waves with the spectra. The emission spectra of atoms in all the wavelengths in the visible range, a ray the gas phase, on the other hand, do not show of white light is spread out into a series of a continuous spread of wavelength from red coloured bands called spectrum. The light of to violet, rather they emit light only at specific wavelengths with dark spaces between them. red colour which has longest wavelength is Such spectra are called line spectra or deviated the least while the violet light, which has shortest wavelength is deviated the most. atomic spectra because the emitted radiation The spectrum of white light, that we can isidentified by the appearance of bright lines see, ranges from violet at 7.50 × 10 14 Hz to in the spectra (Fig. 2.10 page 45). red at 4×10 14Hz. Such a spectrum is called Line emission spectra are of great interest in the study of electronic structure. continuous spectrum. Continuous because violet merges into blue, blue into green and Each element has a unique line emission so on. A similar spectrum is produced when spectrum. The characteristic lines in atomic a rainbow forms in the sky. Remember that spectra can be used in chemical analysis to visible light is just a small portion of the identify unknown atoms in the same way electromagnetic radiation (Fig.2.7). When as fingerprints are used to identify people. The exact matching of lines of the emission electromagnetic radiation interacts with matter, atoms and molecules may absorb spectrum of the atoms of a known element energyandreachtoahigher energy state.With with the lines from an unknown sample higher energy, these are in an unstable state. quickly establishes the identity of the latter, For returning to their normal (more stable, German chemist, Robert Bunsen (1811-1899) lower energy states) energy state, the atoms was one of the first investigators to use line spectra to identify elements. and molecules emit radiations in various regions of the electromagnetic spectrum. Elements like rubidium (Rb), caesium (Cs) thallium (Tl), indium (In), gallium (Ga) and Emission and Absorption Spectra scandium (Sc) were discovered when their The spectrum of radiation emitted by a minerals were analysed by spectroscopic substance that has absorbed energy is called m ethods. The element helium (He) was an emission spectrum. Atoms, molecules or discoveredinthesunbyspectroscopicmethod. ions that have absorbed radiation are said Line Spectrum of Hydrogen to be “excited”. To produce an emission When an electric discharge is passed through spectrum, energy is supplied to a sample by molecules dissociate heating it or irradiating it and the wavelength gaseous hydrogen, the H 2 and the energetically excited hydrogen atoms (or frequency) of the radiation emitted, as produced emit electromagnetic radiation of the sample gives up the absorbed energy, is discrete frequencies. The hydrogen spectrum recorded. consists of several series of lines named after An absorption spectrum is like the their discoverers. Balmer showed in 1885 on the basis of experimental observations photographic negative of an emission * The restriction of any property to discrete values is called quantization. Reprint 2026-27