What is the internal motion of atoms called?

The random fluctuating movements of atoms can be described by the model called Brownian movement or Brownian motion. The phenomenon asserts that with a great number of particles present, Brownian movement will cause the particles to disperse themselves evenly throughout their medium.

Botanist Robert Brown first observed the random motions of pollen on the surface of water under a microscope in the early 1800s. His work was validated in the early 1900s by Albert Einstein, who proved that the motion Brown had observed was caused by the random movements of the molecules of water within which the pollen was suspended.