How do you know that matter is made up of particles?

Answer 1

Rutherford's experiment started it:

He passed alpha particles (which later were found to be helium nuclei, but he didnt know that) through a very thin sheet of gold; about the thickness of a thin tissue. He then measured how many particles passed through the sheet directly (without hitting anything) and how many passed through, hit something and reflected off at various angles. In doing this last step, he checked many different angles: 0, 30, 60, 120,180,240,300, 360. This shows he was a good scientist; he was thorough, precise and made no assumptions on what angles have the results he is looking for.

What he found is that 0 degrees (straight through) had the most flux (number of particles per area), but that 60 and 300 had a good chunk of flux as well. This is expected and confirmed his theory called "Rutherford scattering". But what he didnt expect was that 180 (straight back) had a good chunk of flux too.

This meant that most of the particles went straight through and thus the sheet of gold (which he cannot see through) is composed of empty space. But that sometimes alpha particles hit something and came right back at him.

So the gold sheet is mostly empty space, but with small, heavy objects evenly spaced through it. These are the "particles"; which comes from "particula", the latin word for "little part". We know that the particles in the gold are heavy because the alpha particles have a lot of energy (previously known from other experiments). And when you hit something with a fast/heavy something else and it doesnt move, but reflects it back, then that something is heavier.

So this shows that gold particles are heavy. Looking at the Periodic Table, this is well known by the high atomic number/mass.