Well if they were wood particles it would look like particleboard. If they were wheat particles it would look like flour.

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As in you stacked them in a line, like a one meter tall tower of individual molecules? You'd still see nothing except MAYBE if you were looking straight at the line.

Yes, if you take a massive amount of particles of a given substance, it makes that substance.

And when I say “a massive amount”, chemists used Avogadro’s Number, 6.022x10^23 units, to represent a mole of a substance.

The mass of Moles are generally measured in grams per mole.

So, that’s 602,200,000,000,000,000,000 units of something weighted in grams.

About 4 peas equal 1 gram.

It does depend a lot on the substance though. Like water, as a fluid. Flows together as looks very homogenous.

But if you’re talking about a mixture of various particles that may not blend it makes things very different. You’d likely see a few different substances (like when you smashed different colors of playdough together).

If you made a stack of the particles next to or on top of one another, then you still wouldn't see them because the whole stack would be only 1 unit wide. If you had a pile of a significant width, the particles would probably be easily disturbed by wind and rise up like smoke. If the substance could burn, it would catch fire and explode easily, or settle down and cover surfaces like soot.

It depends (a little bit) by what you mean by particles but significantly more by what you mean by stacked, and also what material the particles are.

Let me give you an example. A meter stick is a 1-meter tall stack of atoms. Atoms are waaaaaay smaller than microscopic. You can see a meter stick, right? But, if you look end on to a meter stick, you see that it is, maybe an inch wide by a half cm tall. So there are already quintillions of atoms just in every layer of the stack.

So maybe you mean a single particle stacked on another single particle... All the way up to 1 m tall. This would be a very very unstable beam. Like balancing a km long pencil probably. But what kinds of particles. If they are just barely too small to see with the naked eye, then even two of them should become just barely visible.

If instead, they are significantly too small to see with a naked eye, then stacking them 1 wide will never become visible. Our eyes can't distinguish things smaller than a certain visual angle, so if the particle stack itself is smaller than that, we won't see it.

There are some notable curiosities tangentially related. Gold can actually form an opaque layer at only a few atoms thick. That's why gold plating is so useful, and gold leaf exists. It's not a 1 m tall stack, but you could say it's a few nanometers tall if you're looking at thickness, or you could say it's however wide and long the object being played is, but then we're going back to the particles being stacked in two different directions that is now visible because obviously it would be.

Thank you for your in depth answer. I'm not a native speaker and "stack" was the wrong term I guess. I meant a pile or something ( what we call " un tas" in french). Like if I take billions of too-small-to-be-seen particles and dump them together.

Right. If the particle you're looking at has a very similar refractive index as air (or if the pile is in water, then the refractive index of water. Whatever medium your pile is in) then you are not likely to see any visible aberrations at all. This needs to be close like withing a millionth of a percent.

If your particle has a significantly different refractive index, but is still mostly transparent, then you are likely to see it clearly. Even if a contiguous mass would be near transparent, every time the light passes from one substance to the other (and a pile like this is likely to have millions of such interactions) then it will scatter a bit of light every time. You will see this most likely as a white powder.

The only other option is that the particle is not transparent. It either absorbs or reflects most visible light, in which case it will look like any other pile of stuff. It will take on the visual properties of that stuff. Except reflection might end up looking more scattered or matte than normal. As an example, a really shiny metal that you could see your reflection in when atomized is likely to lose the perfect surface finish that gives it that specular reflection where an image can be made out. It is going to reflect the light in all directions equally so it will just look like a pile of silver dust.

You are unlikely to be able to create such a string/stack or whatever you wish to call it of a single atom thick string. If you could, you won't see anything. The diameter of an atom is about 1/1000th the wavelength of visible light.

There is the odd case of graphene. It is planar (two dimensional) unlike your essentially one dimensional example. Graphene is a molecule/substance that is a single atom thick consisting of carbon atoms in hexagonal sheet. You can actually see graphene, although the sheet would be much more than a single atom width...