Why Do Atoms exhibit Line Spectra?
Every atom displays spectrum of its kind. Sodium, hydrogen, or any other element shows spectrum characteristic of their respective atoms. Atoms that exist alone have the nature of either emitting or absorbing electromagnetic radiation consisting of energies related to the corresponding structure of the atom. When this radiation is allowed to pass through the prism, it will separate based on the wavelengths called as spectrum. This spectrum can be of three types called as continuum, emission and absorption spectra.
Atoms of gases can have electrons in the excited states. In a group of gaseous atoms, discrete energies possessed by the photons are released when the electrons return back from higher energy levels to lower levels. The transition of electrons releases energy packets which are emitted as emission spectrum. Continuum spectrum is given by those gases which are under high pressures and also given by solids, liquids which when heated releases energy in all wavelengths. This light or radiation when passed through spectrum will appear as continuum spectrum. When light passes through a cool gas which has its atoms in diluted form, the gaseous atoms absorb light and re emit in another direction. This light when passed through prism will give rise to absorption spectrum. So there appears a series of dark lines in the direction of light absorption.
Emission spectrum consists of series of colored overlapping lines characteristic of a particular atom type. In the same way absorption spectrum consists of series of dark lines characteristic of that particular atom type. Therefore atoms exhibit line spectra. When the electrons in an atom fall to lower energy levels, each atom has energy levels in a characteristic pattern and hence emits a characteristic energy radiation pertaining to that atom.
Now due to the electronic transitions the light that is emitted or absorbed can fall in visible wavelength region or UV region or infrared region. The light wavelength that falls in the visible region can be characteristically determined by using prism. The line spectra called Balmer series, Lyman series, Paschen and Bracket series are observed in hydrogen atom line spectra.