Text: "In a diatomic molecule, the distance between atoms is not fixed. There is an equilibrium separation corresponding to a minimum in potential energy. The actual magnitude of the interatomic distance oscillates about the equilibrium distance (point of mutual attraction) (see Keely's work on Three Rotating Bodies) with a motion that is approximately simple harmonic, if the energy of the oscillation is small. "In a polyatomic molecule or in a crystalline solid, the atoms also vibrate about equilibrium positions. For a description of this phenomenon in crystalline solids (see LATTICE VIBRATIONS, also THREE ROTATING BODIES.) Because of the large number of degrees of freedom, however, the situation is more complicated. For example, in the carbon dioxide (CO2) molecule neither oxygen atom by itself moves harmonically, or even periodically. On the other hand, the motion of the CO2 molecule can be analyzed into a number of independent motions, called normal modes, each of which is by itself simple harmonic. In one such motion, for example, the two oxygen atoms move in phase toward or away from the carbon atom. (see diagram) The actual motion of the atoms in a molecule is a superposition (see LAW OF SUPERPOSITION) of the various normal modes and a rotation of the molecule as a whole."

See Also: MAGNETISM, MOLECULE

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