Brownian Motion
Definition and meaning of Brownian Motion in chemistry.
Brownian motion is the continuous, random, zigzag movement of microscopic particles suspended in a fluid, caused by countless unequal collisions with the fast-moving molecules of the surrounding medium.
In more detail
Botanist Robert Brown first observed the effect in 1827 while watching pollen grains jitter in water, but could not explain it. Albert Einstein worked out the theory in 1905, showing that the erratic path arises because a suspended particle is struck far more often on one side than the other at any instant, purely by chance, and that the displacement over time depends on temperature, particle size, and fluid viscosity. Jean Perrin's later experiments confirmed Einstein's equations and gave the first solid experimental proof that atoms and molecules really exist. Brownian motion is the microscopic origin of diffusion and keeps small colloidal particles from settling out under gravity.
Key facts
| Field | Physical Chemistry |
|---|---|
| First observed by | Robert Brown, 1827 (pollen grains in water) |
| Theoretical explanation | Albert Einstein, 1905 |
| Governing relation | Einstein–Stokes equation for the diffusion coefficient |
Smoke particles viewed in a smoke cell under a microscope dart about randomly and independently of one another as air molecules bombard them from all directions.
Frequently asked questions
What actually causes Brownian motion?
Random thermal motion of fluid molecules produces unbalanced, momentary collisions on a suspended particle; because the number of hits from each side fluctuates randomly, the particle is nudged in a constantly changing direction.
Why was Brownian motion important to chemistry?
Einstein's 1905 analysis, confirmed experimentally by Jean Perrin, gave the first quantitative, verifiable evidence that matter is made of discrete atoms and molecules, settling a major debate of the era.