How is this evidence for the existence of atoms?

Just been reading about the history of our understanding of atoms on wikipedia. The article talks about two historical competing schools of thought in regards to the composition of matter:

  1. Made up of tiny indivisible units called atoms.
  2. All matter is “continuous”, can be broken down in to smaller and smaller pieces forever.

The following is then presented as evidence for the existence of atoms:

I can’t see how any of this is evidence for the existence of atoms? (I mean in the context of a debate between atoms vs continuous matter).

I think your trying to understand how that is evidence of what we know an atom to be today and it’s not physical evidence a construct of neutron/protons and electrons - the first experiement simply enforced a mathematical theory about how elements combined. The second simply showed that stuff was made up of elements. And more specifically with the water that a given amount of water would always break down to the same amount of hydrogen and the same amount oxygen - indicating consistancy in elemental make up and structure. We’re really talking about the identification of elements and defining one particle of an element as an atom.

If hydrogen, oxygen and water are composed of atoms, with water consisting of hydrogen and oxygen atoms combined together in some way, then it makes sense for the compound (water) to consist of molecules with a fixed number of hydrogen and oxygen atoms. (We now know it’s two H atoms to one O atom, but that wouldn’t have been known then). That means that water is always composed of the same proportion of the two elements.

If matter were infinitely divisible, it would be hard to imagine what sort of mechanism would cause this property of water (and of other compounds).

I think that the argument is the following:

If matter is continuous, then hydrogen and oxygen are just pure hydrogen and oxygen ‘stuff’, infinitely divisible

Given the above, if water can be broken down into hydrogen and oxygen, then it’s essentially just a mixture of the above two elements.

So why would all samples of distilled water be always mixed in the same proportion, one part hydrogen to four parts oxygen by weight, or whatever it comes out to? If matter is infinitely divisible, then shouldn’t there, somewhere, be a sample of SOMETHING that’s one part hydrogen to three parts oxygen by weight? Why can’t we ‘mix in’ enough hydrogen into water to get such a substance? Why, even if we mix oxygen and hydrogen in that mixture and provide a spark, do we still get water vapour that can be broken down in the usual ratios, and a little leftover water vapour?

It isn’t a proof, but I would call it evidence, considering that the continuous matter hypothesis was unable to advance any explanation for things such as this, and the atomic hypothesis was able to present the idea of molecular compounds, that always contain the same number of the same types of atoms, as with H2O

Does that help?

Triple simulpost for the win?:wink:

That is a great explanation. I get it now, thank you.

On reread, that should be ‘a little leftover hydrogen.’ :smack:

Glad it helped anyway!

As late as the end of the nineteenth century, there were holdouts who insisted that although chemical compounds might behave as though they were composed of atoms, that no one had produced absolute proof that there really were tiny little spheres combining with each other. In addition to being famous for Relativity and the stimulated-emission effect lasers are based on, Einstein gained scientific standing for producing a mathematical analysis of Brownian motion that showed it was exactly what you’d get if matter really was composed of tiny little billiard balls bouncing against each other. (Ironically, just in time for the classical picture of atoms to be outmoded).

And it was for his work on Brownian motion that Einstein got the Nobel, right? He whiffed on Special and General Relativity.

No, but you’re right that he didn’t get it for Relativity – he got it for his explanation of the photoelectric effect, using the idea of quantization.

To the OP, another perhaps illuminating experiment is Rutherford’s scattering experiment, in which he exposed a thin gold foil to alpha radiation – if matter were something continuous, the radiation should just pass right through, or possibly be absorbed in some fraction. However, what was found actually is that in most places, the radiation got through unhindered, but in certain small, extremely localized regions, it was suddenly deflected – as if having hit a small ball of stuff that’s surrounded mostly by emptiness.

Of course, when Rutherford performed this experiment, the atomic nature of matter was well established – what he showed was that the then-prevailing model of the atom as something like a ‘plum pudding’ – a positively charged ‘dough’ dotted with negatively charged electrons – was wrong, and that, instead, almost all of the mass of the atom is concentrated in an extremely small, positively charged nucleus, which is surrounded by a cloud of electrons.

Sorry, just realized I misread the question as ‘What is the evidence for the existence of atoms?’, but it’s too late to edit.

As I understand it, the guy who really clinched it for atoms, by experimentally confirming Einstein’s theory about Brownian motion, was French physicist Jean Perrin.*

I am not sure offhand when Perrin’s work was first published, but it must have been after Einstein’s 1905, and as the Rutherford-Geiger-Marsden scattering experiment was published in 1909, I doubt that it is true to say that the existence of atoms was “well established” by then. However, Rutherford and his boys probably worked on the assumption that atoms are real. The skeptics were a minority, albeit a vocal one, well before Perrin’s or Einstein’s time
*See: Mary Jo Nye: Molecular Reality: a Perspective on the Scientific Work of Jean Perrin. Elsevier, 1972.

My recollection is that Perrin’s work clinched what was, in fact, a well established theory. It also gave a rather precise value for Boltzmann’s constant and thus established Avogadro’s number and unequivocally established the size of atoms.

The other aspect of Dalton’s work that made the atomic hypothesis so compelling was that he could attribute a mass to each atom (now measured in Daltons). In examining chemical reactions, the results could always be explained using integer ratios of reactant and product molecules, always using the same value for the mass of each element. This was the real beginning of modern day chemistry.

Fascinating. As one of the resident physicists I’d like to offer the contribution that there is pretty good proof here that atoms do NOT exist.

That is, the OP quote is relevant to the concept of some little unit that cannot be subdivided, whereas the things we are now used to calling “atoms” can be (the computer you are looking at wouldn’t work if the electrons couldn’t be shifted around in solids, for example).

What I mean is that there are two entangled lines of reasoning about two different base concepts. One of the base concepts is about ultimate building blocks of matter. The other is about a tier of organization that creates the rules underlying chemistry. The idea of the “atom” has gradually drifted from the first to the second.

This discussion is partly about elements, and proving that chemistry hangs on the fixed nature of a finite assortment of elements that react with one another, and that the elements are available only in quantized doses.

But the whole idea of atoms was supposed to be that they are the ultimate building blocks, whereas this is certainly untrue in several contexts.

which is why the composition ratio is evidence, not proof. It is easist explained with the atom model, but you can dream up other explanations - just not as simple or logical.

Similarly - regular crystalline structures are explained as coming from an assembly of tiny basic units with the appropriate geometric “connectors” - evidence, but not proof.

Oddly enough, my old edition of the Encyclopedia Britannica had this to say, in an article about the silicates:

Now that’s a tread title!