Motion of Molecules

All molecules are in constant motion. Molecules of a liquid have more freedom of movement than those in a solid. Molecules in a gas have the greatest degree of motion.

Heat, temperature and the motion of molecules are all related. Temperature is a measure of the average kinetic energy of the molecules in a material. Heat is the energy transferred between materials that have different temperatures. Increasing the temperature increases the translational motion of molecules Energy is related to temperature by the relationship:

E= kT

Click on the image below and watch the motion of water molecules at room temperature. The time scale is on the order of picoseconds (10 -12 seconds).The green lines represent hydrogen bonds between water molecules.

Internal motion within molecules

If your computer will run Java Applets view the complete motion of a molecule.

Click on the image

Be patient. It may take several minutes for the applet to completely load.

Look for the three basic types of motion within the molecule.

  • bond stretching
  • bond bending
  • torsional motion

    The following MPEG shows similar molecular motion for a smaller molecule. The movie emphasizes the torsional motion within the molecule (see left side of movie). It is shown that the torsion bond energy of a molecule can be modeled by a periodic function.

    753K Movie

    Molecular motion is also affected by attractive and repulsive forces within the molecule known as Coulombic Forces.

    1066K Movie

    The movie above shows two particles of opposite charge attracted to each other. The relationship between coulombic energy and distance is an inverse relationship. The coulombic energy between the two particles varies with -1/r, where r is the distance separating their center of masses. In the movie notice that as r gets smaller, the potential energy becomes more negative.

    For more advanced lessons on molecular motion click here