What Is Molecular Motion?

Translational and rotational motion, as well as bond stretching and bending, are the basic types of motion within molecules. Different molecules undergo different superpositions of the basic motions.

All molecules are constantly in motion. The degrees of freedom of this motion depend on the molecules' phase of matter. Solids have the least range of motion, followed by liquids, then gases. The temperature of the molecules is an indication of their kinetic energy. Higher temperature directly increases the translational motion of the molecules, leading to higher velocity of motion in liquid and gas molecules and higher frequency and amplitude of vibration in solid molecules.

The Boltzmann relation linearly relates macroscopic temperature to molecular kinetic energy through the Boltzmann constant. A linear increase in temperature is accompanied by a linear increase in the average kinetic energy of all molecules in a system.

Rotational motion can involve the rotation of the entire molecule or the rotation of parts of the molecule relative to one another in torsional motion. Such motion can only occur in gas and liquid phases, as molecules are bonded in fixed positions in solids.

The motions of molecules determine many physical properties including color, which is determined by photon absorption and re-emission through the bonds, and chemical reactivity, which is determined by the amount of energy needed to form or break the bonds.