Absorption spectrum and emission spectrum

Why does an atom absorb only light of particular wavelengths? Why does it then emit only light of these same wavelengths?

Absorption spectrum   

Every shell (electron shells and transitory shells) of an atom has its own unique natural frequency (or resonant frequency). When a cooled gas is placed in the path of continues spectrum of light, dark absorption lines will be appeared in the resulting spectrum. This absorption lines are caused by the absorption of that particular frequencies by the gas atoms.

When the natural frequencies of electron shells and primary transitory shells of an atom are same as the frequencies of some of the incoming photons, then that photons excite their corresponding shells (electron shells and transitory shells) and in the resulting spectrum that photons will be absent.

Emission spectrum: If the same gas is examined at an oblique angle, bright emission lines will be visible against a dark background. When a photon is absorbed by an electron, the electron makes oscillation in the same frequency of the absorbed photon and the photon is recreated. These recreated photons by the gas atoms are the emission lines.

If we can make the whole of the absorption spectrum lines or emission spectrum lines of an atom in all frequencies from Infrared rays to X-rays at its cooled state, we can directly observe the natural frequencies of each of the electron shells and the primary transitory shells of that atom.  The total number of electron shells and the primary transitory shells in an atom is equal to the total number of absorption or emission spectrum lines. The natural frequencies of the electron shells and primary transitory shells are, if we consider the emission lines, then the high frequency line for the innermost electron shell and the low frequency line for the outer most primary transitory shell. Same as, if we consider the absorption lines, then the high frequency absorption line for the innermost electron shell and the low frequency absorption line for the outermost primary transitory shell.