Random motion, also known as Brownian motion, is the chaotic, haphazard movement of atoms and molecules. Random motion is a quality of liquid and especially gas molecules as described by the kinetic theory. The botanist Robert Brown first noticed this phenomenon in 1827, and Albert Einstein later continued Brown’s study of the movement of water and pollen molecules. His theory helped to definitively prove the existence of atoms.
Liquid and gas molecules, unlike solids, do not remain in a fixed position. Solids have the least amount of energy of the three states of matter, and thus, their molecules only vibrate slightly. The molecules in liquids have more energy and move around each other in random order. Gases have the most kinetic energy; their molecules are arranged far apart and move speedily in all directions, occasionally even bouncing off of each other. This difference in motion between solids and liquids and gases is best illustrated by putting the molecules in a sealed container. A solid does not change shape, but a liquid flows to assume the shape of the container. Because the molecules of a gas are in constant random motion, the gas spreads out to the very edge of the container and completely fills it.