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

Question

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

Answer 1

They are too small to see with our eyes, but that doesn’t mean we can’t see them in other ways.

When we take pictures we usually rely on photons, on visible light. Visible light has wavelengths between 380 to about 750 nanometers (there are 1 billion nanometers in one meter, or 25.4 million nm in one inch.). This works because most objects we take photographs of, such as plants or animals or people, are much much MUCH bigger than 380-750 nanometers.

But atoms are just too small, ranging in size from 50 picometers for one helium atom to 520 picometers across for one cesium atom. A picometer is 1 trillionth of one meter, meaning its 1000 times smaller than a nanometer. You could fit 730 cesium atoms in a row in the same length is the SHORTEST visible wavelength of light.

What that means is visible light never/almost never reflects off atoms. It would be like trying to detect if there was a grain of sand by bouncing a basketball off it. The odds that you’d hit the sand with the basketball are pretty tiny!

So if we want to take a “picture” of an atom we have to use something smaller than the atom. Fortunately we have something that we can use for that, electrons. Electrons are much smaller than atoms (though their exact size is up for debate), the range is between 10^(−18) and 10^(−22) meters. For comparison, the smallest atom (helium) is about 5*10^(-11) meters, meaning its 10 million to 100 billion times bigger than an electron. By using precisely controlled streams of electrons along with electron detection devices we can detect atoms and use the measurements we make to render images of them that we can then see.

Meanwhile electrons can be detected and controlled using magnetic fields because all electrons have a negative charge. So even if we can’t “see” them we can know there are there. Kind of like how you can know someone is in the room by smell or sound even if its pitch black and you can’t see them with your eyes.