if atoms are too small to see, how do we know atoms exist? And who discovered atoms?


This was a real question by my 6yo. She asked what was made of atoms. I said everything, but they’re too small to see. “Oh, microscopic!” “Well, smaller actually.” “How do people know they exist? And what’s the name of the person who discovered them?”

Marked as chemistry cause that’s how ill-equipped I am to answer this question. Is it chemistry? Or should I have said physics?

In: 4

We can’t see them directly but we have models that predict their behavior and we have experiments to test the model.

It’s like if I was in a pitch black room and I throw a basketball at the wall. I can’t see the wall but I’d expect the ball to bounce back if there is a wall.

No idea who discovered atoms though.

Early experiments in physics led to the precursor of the standard model



The development of atomic theory goes back to a few Greek philosophers who proposed the idea that there were limits to physical division; at some point you couldn’t keep dividing something, and it would be ‘atomos’, meaning indivisible.

This idea was largely seen as ridiculous and most people ignored it for about 2000 years.

In the early 1800s, John Dalton (like other chemists of the time) was experimenting with various chemicals, and noted that particular combinations would always have the exact same weight ratios in their reactions. He theorised that they were discrete combinations occurring at vast scale, and called the units of these atoms, after the old Greek idea.

Further proof for atomic theory came from Robert Brown, who noticed in the 1820’s that tiny particles in water would jiggle for no reason. It took until Einstein in 1905 for a formal explanation to arise – they moved because the water was actually made from tiny particles that were bouncing around.

In short, we considered the evidence available and eventually concluded that all of it could be explained by the existence of atoms.

These days, we can actually image individual atoms with the use of electron microscopes.

No single person discovered atoms. Historians characterize atomic theory as going through several distinct phases:

1. Philosophical atomism (ancient Greeks) — an approach that was the _idea_ of atoms as an abstract question about the world (basically one the answer to the question, “if you cut something in half, and then cut the remainder in half, and cut the remainder in half, would you eventually end up with something that couldn’t be cut?”) than any physical reality of them. It was edged out by Aristotle’s (non-atomic) theory of matter (plenism) for several thousand years.

2. Chemical atomism (18th/19th century product of the chemical revolution) — mainly attributed to Dalton, who basically said, “hey, this new approach to chemistry makes sense if you imagine each element is an atom and that these atoms can be combined into molecules. A lot of the people who practiced this did not know if atoms really existed or were just convenient abstractions.

3. Physical atomism (late 19th century through 20th century) — the creation of what we think of as the modern idea of atoms as collections of subatomic particles with a real physical structure. The discovery the electron (Thompson, 1897) as the first subatomic particle (Thompson believed it was the _only_ one) was used to start new work and speculation about atomic structure that, over the course of several decades, led to the understanding we have today.

As for how people know they exist: there are lots of experiments whose results wouldn’t make sense if atoms didn’t exist. There was no single experiment to establish their existence; it gradually just became assumed that they existed, because it made far more sense to imagine they existed than the contrary.