According to Quantum Mechanics, the electrons in every atom of a given element (say, hydrogen, oxygen, iron, whatever) are permitted to move in certain distinct orbits around the atomic nucleus, characterized by the energy of the electron or the orbital frequency. You can think of these permitted orbits as the natural resonance frequencies of the atom, just as each different wire of a piano (or pipe of an organ) will vibrate at only one tone or its overtones. We can therefore recognize an atom by the wavelengths of light that it emits, just as we can recognize which key of a piano has been struck by the tone of the sound the piano emits.
Atoms can be excited by either collisions with other atoms or electrons, or by the absorption of photons. When an atom is excited, one of its electrons knocked into an orbit of higher energy. The electron will stay in the excited orbit for a while (typically, about a billionth of a second), and then will suddenly drop back into an orbit of lower energy, spitting out an emission line photon as it does.
When atoms are chemically bonded in a molecule, the electron orbits of the molecule are different from those of either atom and we can recognize the molecule by the spectrum of radiation it emits and absorbs. Atoms in molecules can also tumble and vibrate. As a result, the spectra of molecules are more complex and richer than the spectra of atoms. Also, molecules, unlike atoms, often have many emission and absorption lines in the infrared and radio bands of the spectrum.
The CU Physics Department has a nice animation showing how an electron orbiting an atomic nucleus absorbs a photon and jumps to another orbit. After a little while (about 10-8 s), the excited electron drops back to the original orbit and spits the photon back out. In the real case, unlike this animation, the electron will orbit about a million times before it drops back. Also, the wavelengths of the absorbed and emitted photons will be thousands of times greater than the orbital radius. You should also check this second animation showing how X-rays are made. A fast electron (produced by high voltage) strikes an atom and knocks out an inner electron. Then the outer electron falls into the orbit of the inner electron, emitting a photon as it does.
You may find an excellent review of these topics (with some good graphics) in Nick Strobel's Astronomy Notes.
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Last modified October 18, 2002;
Copyright by Richard McCray