What Dalton Discovered
Notice the big old gap between the time when Democritus first shouted, "Atomos! We've got atomos here!" to the publication of Dalton's more formal theory? Told you Aristotle threw us back into the atomic dark ages a bit. It's not that nobody was thinking about atoms at all for nearly 2000 years (check out that expanded timeline), but what Dalton discovered and what he formally published about atoms earns him the next slot on our field trip through the history of atomic models.
His Experiment: Observing Chemical Reactions
Dalton spent a lot of time in his lab observing various chemical reactions. By looking at how things reacted and recombined to form new substances, Dalton was able to build on Democritus' idea of atoms as the fundamental building block's of matter and go further to say that there were many different "flavors" or kinds of atoms.
His Model: Billiard Balls
Where Dalton and Democritus would have agreed (if they hadn't been separated by 2000 years) is that atoms were the smallest, most basic unit of matter. Indivisible into smaller parts. (Of course they were both wrong, but we'll get to that later). Where Dalton advanced atomic theory was by saying we had many different atoms out there. His model, often dubbed the "billiard ball" model, basically says you can't divide the atom into smaller pieces.
Dalton also came up with some very important things to know about atoms and how they combine. Most of these still hold true!
- All matter is made up of atoms, and these little guys are indivisible (can't break them apart) and indestructible (can't break them down). He was half right.
- All atoms of a given element are "the same" (okay, careful here - remember you know more than Dalton did. "The same" still works for number of protons, but now that we know about isotopes it's hard to say all atoms of an element are "the same" if by that you mean identical.)
- Compounds form when two or more different types of atoms bond chemically, and they do this is predictable, fixed ratios.
- Atoms are not destroyed during chemical reactions, they are just rearranged a bit.
Okay, so today we know atoms are made of smaller parts, and you can break them down in nuclear (not chemical) reactions, but overall Dalton set up a great base of knowledge for the next group of atomic scientists to build on.