Q&A: First Proof That Atoms Were Real

Question: At what point did scientists realize that atoms and molecules were real things? When I read your post about how we learned that outer space was a vacuum it made me wonder about some other scientific facts. I’m guessing it was sometime after microscopes were invented? — JH, Hattiesburg, MS

[JH is referring to my post: https://sky-lights.org/2021/06/14/qa-how-we-knew-space-was-a-vacuum/]

Answer: You are correct about the timeline. The existence of atoms was effectively demonstrated in 1827, and proved theoretically in 1905. But they could not be seen with the optical microscopes available at that time. It’s an interesting story that goes as far back as circa 400 BC to the teachings of Democritus.

His theory posits that if you divide something, say a stone, into halves, and then divide it again repeatedly, you must at some point reach a “smallest possible particle” of stone that itself cannot be divided. He called that smallest piece atomos (Greek for “indivisible”). He further hypothesized that between the atomos there must be empty void where nothing existed — a controversial idea at that time. And that’s where the concept of atoms stood for nearly 2000 years.

The first scientific evidence emerged in 1827 when botanist Robert Brown observed pollen floating in water using an optical microscope with a magnification of 170X. What he saw was the kind of motion you see in the video (although this video shows smoke particles suspended in air). The phenomenon became known as Brownian motion. And as you suspected, this was over 200 years after the invention of the microscope.

The video shows visible smoke particles around 2.5 μm in diameter (PM2.5) being jostled around by invisible air molecules around 0.3 nm (0.0003 μm) in diameter. The smoke particles are some 8200X larger than the air molecules jostling them.

Then, in 1905, theoretical physicist Albert Einstein published a paper that explained the random motion of the pollen particles. Molecules of water in which the pollen was suspended are themselves in continuous random motion. This motion was an essential component of the kinetic theory of gases. Einstein proved that, for a sufficiently small particle (like a pollen grain), the number of water molecules striking one side of the particle (compared to the other side) would be unequal enough to move the particle in some random direction. It was a statistical argument, but it was convincing proof for the cause of Brownian motion — and for the existence of molecules and, by extension, atoms.

For larger particles, say a wooden bead, there are so many water molecules colliding that the statistics average out and no jostling is observed. There would still be more molecules hitting one side than the other, but the imbalance would be much smaller compared to the total number of molecules involved.

Here’s a great simulation showing what’s happening. The large yellow particle is a pollen grain, and the smaller black ones represent air (or water) molecules:

[This simulation was created by Francisco Esquembre, Fu-Kwun and Lookang, and is available on Wikimedia Commons at: https://commons.wikimedia.org/wiki/File:Brownian_motion_large.gif]

It’s often said that “seeing is believing” but we can’t really do that with atoms and molecules. When we “see” objects our eyes are receiving photons of light that were reflected from, or emitted by, that object. Unfortunately, atoms are much smaller than the wavelengths of light our eyes can detect. Violet light has a wavelength of around 380 nm, and the largest atoms are around 0.175 nm. If you had a large atom suspended in space, and shined a bright light at it, the photons would simply diffract around the atom and not reflect.

Watch this wave tank demo from an old physics film loop. Starting around 1:10, they begin to gradually increase the wavelength impinging on the object. When the wavelength is smaller than the object, you get an obvious “shadow” behind it, and reflections from the front, i.e., the object is “visible”. But when the wavelength becomes larger than the object those effects disappear and the waves just diffract around the object, rejoining downstream with no loss of energy.

So we can’t bounce light off an atom. But we can bounce electrons off them since electrons are essentially “points” compared to atoms — their classical radius is around 0.000003 nm. Below is an electron microscope image of graphene. I call it an “image” instead of a “photo” because it is digitally constructed from info about how many electrons are bouncing back to a detector as the electron beam raster scans the sample:

The electron microscope was invented in 1931 by Ernst Ruska, a German electrical engineer. Today’s electron microscopes provide better resolution and higher detail, and images like the one above. It’s still a bit fuzzy, but the six carbon atoms that make up each graphene ring are obvious. And that’s probably the best we can do, as atoms and molecules are more quantum in nature and not “solid” like things appear at macroscopic scales.

As with proving space is a vacuum, proving atoms and molecules exist was a long circuitous journey with contributions from many areas of science. Chemistry basically took them for granted since it was the only way to explain how different substances can combine. Personally, I think the first real proof was Einstein’s statistical analysis of Brownian motion.

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