The Great Story of Atoms
We all have learned, somehow or other, in middle or high school, that the Universe is made up of atoms, that bind each other to form the matter that surrounds us. However, have you ever wondered how we discovered their existence? If you thought it is by observing them, you were wrong!
The story of atoms is a tortuous story full of meanders that began with Robert Brown, a Scottish botanist who, while studying pollen grains, discovered that if we suspend one of them in water under a microscope, we can see it skittering around.
This Brownian motion, as we nominate it henceforth as a tribute to its discoverer, was already observed. However, his predecessors took it as being the manifestation of a vital principle present in pollen grains and which is linked with biology.
Nevertheless, Robert Brown, like all geniuses, was more clever than that…By repeating the experiment with particles of inorganic matter, he concluded that the motion is a physical and life-unrelated phenomenon.
Until this stage of the story, the vast majority of the readers will not see the link existing between the mysterious Brownian motion and the proof of the existence of atoms. Indeed, it took over a century to get an answer thanks to Albert Einstein and Marian Smoluchowski who proposed to explain the Brownian motion through the research works of Ludwig Boltzmann on fluid mechanics. The idea: observe their effects rather than the atoms themselves.
Let us be explicit and imagine a microscopic pollen grain suspended in water where there are a plethora of skittering particles. We do not see them nonetheless, but we can observe their effect on the pollen grain which is nothing but its continual and irregular path resulting from the collisions of the invisible water particles against it.
The two physicists managed thereby to formulate the equation that permits calculating the diffusivity of particles:
where “R” denotes the ideal gas constant, “T” the temperature, “η” the dynamic viscosity of the fluid, “r” the radius of the particles and “N” the Avogadro constant.
A breakthrough has been done by now, and the mysterious Brownian motion will not withstand for a long time the shrewdness of the scientists. In fact, there is still a physical parameter not determined at that time; namely the Avogadro constant. As matter of fact, theorists cannot go any further, so experimenters take over from that point. Jean Perrin began then his experimental work trying to determine the diffusion coefficient, and he successfully did it by several different methods.
Let us go back to our equation now. We know all the physical parameter but the Avogadro constant, we can then calculate it. However, what was more astonishing and startling is that the value is always regardless of the nature of the fluid. And as the French saying goes lots of coincidences kill coincidences. Then all physicists around the world admitted the atomic theory without observing any atom.
Rami Benouahmane
