Can Anyone Explain How Atomic Particles Were Described So Early?

I am pretty decent at most science. However, I have a hard time with things that I can’t see in one of many ways. It blows my mind that anyone figured out what an atom was long before most modern inventions. It is even more mind-boggling that people figured out what the particles like the electron, neutron, proton and the atomic nucleus roughly looked like a long time ago. I would ask about the subatomic particles but that is enough for now.

Can anyone give a Carl Sagan or Steven Hawking type answer as to how physicists figured out there was an atom and them expanded on it to include things like a nucleus, proton and electrons so long ago?

You might start by reading about Rutherford’s experiment:

Wikipedia does a pretty good job. Atomic Theory.

It wasn’t until very late in the 19th century that we discovered the electron, and that atoms weren’t the smallest elementary particle. And then it was early in the 20th that we discovered the atom was mostly empty space (Rutherford experiment).

So, I’m not sure we didn’t have a lot of “modern inventions” by then. Also, no one knows what an elementary particle “looke like”, as they can’t be seen.

It’s a mistake to think subatomic particles look like anything. They don’t. They don’t even exist in all of the ways we’re accustomed to. We draw them as little dots because most people can’t read the relevant equations.

It is worth recalling that the atomic hypothesis was not universally accepted even by 1900. Einstein’s explanation of Brownian motion went a long way. But Rutherford’s experiments were definitive. Also, the Millikan oit-drop experiment that allowed him to show that charges on tiny drops were always integer multiples of something, pretty strongly suggested that there were discrete particles, all with the same unit charge.

The existence of atoms was surmised from the experimentally observed fact that chemical elements always combine in specific proportions to make compounds. For instance, water is made up of two parts hydrogen to one part oxygen, exactly - the proportions never vary. This is easily explainable if atoms exist - two hydrogen atoms are combining with one oxygen atom to form one water molecule. If there are no atoms, and substances are infinitely divisible, these exact and unvarying proportions are hard to explain convincingly.

Just a note here, this is only true if the ingredients are gasses, and are measured by volume (at some given temperature and pressure). And even there, there can be complications, since not all elemental gasses contain the same number of atoms. Ozone is pure elemental oxygen, too, but you’ll combine three parts hydrogen with one part ozone to make water.


Um, flex727, I could explain a bit more there, but could you perhaps be a touch more specific about what’s puzzling you? Some questions, maybe, or a line-by-line breakdown?

Chronos, I understand your explanation, but what I don’t understand is why you even bring gases (by which, flex, he means O2(g) and H2(g)) into the picture.

The formula of water is H2O. That’s two parts hidrogens, one part oxygen. This does not take into account how the molecule was “built” (hey, a particular water molecule exists only for a short time as the atoms rearrange around; a particular one could come from dehydration of a molecule of alcohol and five minutes later that O and those two Hs could be centimeters apart). It does not consider the weight or the volume of the components… it does not consider that most water in a glass will be strung up with others because of the hydrogen bonds, nor that some of it will be dissociated in H+ (sometimes written as H3O+) and OH-. Those details are irrelevant to the issue at hand and irrelevant to a lot of the chemistry of water.

flex, Chronos was talking about the recipe to cook water. Of which there are many, and only mixing 2 H2(g) + O2 (g) —> 2 H2O (out of those many recipes) are you mixing “two parts in volume of hydrogen plus one part in volume of water.” For that same recipe, the “parts in weight” are not 2:1.

The first formulation of an atomic theory is from Democritus. He figured that, as wild as it sounded, having more than four elements and having each element formed of “undivisable” (a-tom) particles which could combine with those of other elements to make compounds made more sense than the other theories floating around at the time. But he didn’t have the means to do experimental research on this. While there were troglodytes fighting this until the early 1900, there were also people who had accepted the notion of “many elements” and were actively looking for more of them in the 1600.

Thanks for the more detailed explanation, Nava.

Chronos, I was just a bit slow on the uptake yesterday.

Water is two parts hydrogen to one part oxygen, but how do you measure those parts? If I take two kilograms of hydrogen to one kilogram of oxygen, for instance, I’ll have some hydrogen left over. I could define my parts in terms of numbers of atoms, but that’s circular, since we’re looking for something that could be used to deduce the existence of atoms. What I need is something that macroscopically measurable, but which is proportional to the number of atoms. As it happens, the volume of a gas (any gas, no matter what it is) at a given temperature and pressure is proportional to the number of molecules you have of it, and many gas molecules (including hydrogen and oxygen) consist of exactly two atoms. So if instead of 2 kilograms of hydrogen and 1 of oxygen, I instead take 2 liters of hydrogen and 1 liter of oxygen, I’ll use up both exactly, with none left over.