Final answer:
Brownian motion is the erratic movement of particles suspended in a fluid due to collisions with fluid molecules, observed by Robert Brown in 1827. Einstein's work on the topic allows for the measurement of atomic and molecular sizes. Brownian motion provides direct evidence of the existence of atoms and underlies the process of diffusion.
Step-by-step explanation:
Brownian motion is the random and chaotic movement of particles suspended in a fluid, such as water. This phenomenon was first observed in 1827 by the Scottish botanist Robert Brown, who noted that tiny pollen grains suspended in still water moved in complex, unpredictable paths. The cause of this motion is the collision of the suspended particles with the molecules of the fluid, which are constantly in random thermal motion.
These collisions result in statistical fluctuations in the number of molecules striking the sides of the suspended particle, pushing it first in one direction, then another. Albert Einstein later explained in detail how this motion could be used to measure the size of atoms and molecules, providing a way to determine their size more accurately than had been possible before his theories.
Thus, Brownian motion is not only direct evidence for the existence of atoms and molecules but is also a practical tool for studying phenomena at the microscopic level, such as diffusion. The random to-and-fro movement characteristic of Brownian motion is also a classic example of diffusion, where particles spread out from areas of higher concentration to areas of lower concentration.