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Name:  jessica a gancarski
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Date: 1999 

Who discovered the atom?

A simple question, but deceptively complicated. What does it mean to "discover" something? The first person to propose that matter was made of atoms, and then write it down, was a Greek philosopher named Democritus. But he had no experimental proof of his notion. Then, a number of scientists, starting probably with Newton in the late 1600s, proposed a corpuscular, or atomic, model. But it wasn't until the late 1700s/early 1800s that John Dalton proposed that all matter was made of atoms and actually used it to explain a bunch of experiments that had been done on gases, and to calculate atomic weights of elements. However, he still hadn't PROVED that atoms existed...he just showed that the atomic concept was useful and helped explain a lot of data. Probably the best direct probe of the atom was first done by Rutherford and his student, C.T.R. Wilson, who invented the cloud chamber and used it to show that when thin gold foil is bombarded by helium nuclei (alpha particles), the particles are occasionally deflected by a very large angle, but usually pass straight through. This gave rise to the realization that the gold was composed of atoms, with a tiny nucleus at the middle which could occasionally collide with an alpha particle and send it flying.

-prof topper

Coupla more comments for ya. I think the Greek concept of the atom was unlike ours: to their minds a pickle was composed of small green sour atoms, a fire of hot light bright atoms, etc. More of an aesthetic than empirical concept. In my opinion they get a little more credit in general for atoms than they deserve. Second, in addition to Dalton's work suggesting the atom because of fixed chemical combining rules, there was the astoundingly successful kinetic theory of gases, a subject of intense interest in the nineteenth century, which relies utterly on gases being made of little bits of flying matter. Names here are Bernoulli (1740s), Joule (1850s), Clausius & Maxwell (1860s), and finally the giant Boltzmann (1870s). Even as late as 1900 the existence of atoms was seriously doubted by able scientists (e.g. Mach), and from the point of view of many the definitive proof is the apparently unique explanation by Einstein in 1905 of "Brownian motion," the incessant jiggling of pollen grains visible under a microscope that is caused by its battering by the invisible molecules. The experiment in 1909 by Geiger and Marsden in the laboratory of Rutherford (alpha particle scattering from gold foil) is usually considered proof of the existence of the nucleus, although of course a nucleus implies an atom too.


It should however be pointed out that most scientists utterly rejected the kinetic theory of gases, and that the reputations of all the aforementioned scientists were damaged severely in the eyes of their contemporaries by their espousal of atomic theories. Boltzmann in particular took the rejection of his work particularly fact, he killed himself. Too bad he didn't wait around a bit longer.

I think that you could argue that the early spectroscopic work of Angstrom, Balmer, etc. provided proof of the existence of atoms, although the experiments may not have been properly interpreted at the time. In hindsight, they were probing atomic energy levels.

Finally, although the Greek view of "atoms" was certainly not the same as the idea we have today, certainly that particular school was the first to put forth the idea that the microscopic properties give rise to macroscopic properties. Sure, they got the details all wrong, and worked without experimental data. In retrospect, it was a lucky guess. But science is full of lucky guesses, and in my opinion it does no harm to give Democritus credit for his cleverness. Especially since almost noone believed him at the time....often a good sign that one is right.

-prof topper

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