‘[…] when atomic theory was first proposed, it sounded pretty crazy. And yes, we call it ‘Atomic Theory’, using the scientific definition of theory, which is “a well-tested set of ideas that explains many disparate observations”, not the colloquial definition of theory, which is “a guess.” But luckily there’s no-one running around any more saying “atoms are just a theory.”
But it wasn’t that long ago that people were running around saying that. You want to know who settled it for good? Einstein! Atoms had been postulated for a long time by the 20th century, but it wasn’t until Einstein mathematically proved the existence of atoms and molecules in 1905 that the matter was truly settled. And you thought Einstein was all about relativity and E=mc2, he also proved atoms exist!
Here’s how it happened. In 1827, a botanist named Robert Brown was looking at pollen grains in water through a microscope and he noticed that they jiggled randomly even when there was no movement to cause the jiggling. It was a mystery for a long time, until 1905 when Einstein theorized that this phenomenon was caused by as-yet-unproven atomic particles actually smacking into the grains of pollen.
He wrote up some fancy math, showing that his theory predicted this motion almost perfectly, and everyone had to concede that yes, tiny discrete bits of matter were indeed smacking into the pollen, and thus molecules, and by extension atoms, must exist. Today, we remember this botanist and his discovery by calling the motion he observed Brownian motion.’
– Green. H. (2013, February 11) The Nucleus: Crash Course Chemistry #1