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Atomic Spectra

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Atomic Spectra - Lesson Summary

When a ray of white light from the sun or an incandescent lamp is passed through a prism, it is dispersed into a continuous array of seven colours, namely, VIBGYOR, called spectrum.

It is observed that in a spectrum the radiation with lower wavelength bends more than the radiation with higher wavelength. The colored bands merge into each other therefore; the spectrum of ordinary white light is called continuous spectrum.

Electromagnetic radiations include a range of wavelengths and this array of wavelengths is referred as the electromagnetic spectrum.

When electromagnetic radiation interact with matter, atoms and molecules, it absorbs some part of radiation and go to a higher, unstable energy state called excited state.

As the atoms are unstable in their excited states, they tend to revert to its original state, called the ground state, by emitting the same amount of energy that it had absorbed earlier.

Depending on the amount of energy emitted, a radiation of particular wavelength is obtained.

The spectrum of radiation emitted by an atom is called emission spectrum.

In contrast to the continuous spectrum of white light, an emission spectrum is usually discontinuous with dark spaces between the bright lines, called line spectrum.

The bright lines in the emission spectrum exist due to emitted radiations of specific wavelengths, but not of continuous spread of wavelengths as in the case of white light.

A photographic negative of emission spectrum is the absorption spectrum. An absorption spectrum is recorded by passing a continuum of radiation through a sample which absorbs radiation of certain wavelengths.

The study of emission or absorption spectra to determine the molecular structure is referred as spectroscopy.

Elements like rubidium, cesium, thallium, indium, gallium and scandium were discovered when their minerals were analyzed by the spectroscopic method.

When an electric discharge is passed through hydrogen gas, hydrogen molecules dissociate to produce energetically excited hydrogen atoms.

These excited atoms emit electromagnetic radiation of discrete frequencies while coming back to lower energy levels.

The resultant hydrogen spectrum consists of multiple series of lines that are named after their discoverers.

The first five series of lines that correspond to n=1, 2, 3, 4 and 5 are known as Lyman, Balmer, paschen, bracket and pfund series respectively.Each of these series represents the line spectrum emitted by the electrons of hydrogen atom when they change state from n2 to n1 principal energy levels.

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