Thermal motion refers to the random motions of molecules, atoms, electrons or other subatomic particles. Unlike the visible world around us, the atomic world is in a constant state of motion at all temperatures above absolute zero. The thermal motion of particles rises with the temperature of those particles and is governed by the laws of thermodynamics.
The study of thermal motion is the study of the random motion of particles. Molecules, atoms and subatomic particles do not behave in a predictable manner. Unlike the world we see, these tiny pieces of matter are almost always in constant motion and do not follow the same rules as the larger bodies they make up. Electrons, for example, exist in orbitals around the nucleus of an atom. Though the exact location and motion of an electron cannot be determined, there is a likelihood that they will move around within a certain space, known as an orbital.
Atomic particles remain in constant motion at all temperatures above absolute zero. Absolute zero, also called 0 degrees Kelvin, is equal to -459.67°F (-273.15°C). This is the lowest temperature that exists because it corresponds to the temperature at which atomic particles cease to move.
The thermal motion of a particle is related to the temperature of that particle. Particles at higher temperatures exhibit greater motion than those at lower temperatures. This is true of particles in any state of matter, including gas, liquid, solid and plasma. Though the atoms in a solid are closer together than the atoms in a liquid or a gas, there is still space for the atoms move around.
The thermal motion of atomic particles was first describe by the physicist Robert Brown. When viewing a small particle, such as a grain of pollen or a piece of dust under a microscope, Brown noticed that the particle seemed to be in a constant state of motion, or agitation. The motion of the atoms around a small particle causes the atoms to bump into it. This makes the larger particle move around randomly, just as the atomic particles do. This type of motion is referred to as Brownian motion.
Thermal motion is studied through thermodynamics, which has a set of laws that govern the random movement of particles. The first law states that matter and energy are always conserved. The second, somewhat paradoxically, states that a return to a previous energy state is impossible because some energy escapes from the system and can never be used again. The third states that absolute zero cannot be attained. Simply put, these laws mean the motion is a random motion that never ends and always changes.