Historical question about the atomic theory

[Swammerdami]

I will ask a specific question about a historical development in scientific theory -- although this may be a complicated development; with an arduous answer.

First, a brief history of the atomic theory of matter.

Democritus, a contemporary of Socrates, is famous for espousing a theory of atoms. He was preceded and inspired by Leucippus born ca 20 years earlier. This latter was a disciple of Zeno and Parmenides although AFAIK neither of these espoused atomism. The theory was quite popular, on and off; Titus Lucretius Carus of the 1st century BC was a proponent who argued that Brownian motion was evidence in its favor. Still the theory remained controversial for over two millennia. Sir Isaac Newton was the most famous proponent prior to the rise of quantitative chemistry, 1774-1807.

By the early 19th century chemists led by John Dalton were reducing compounds to their elements, and comparing the weight ratios. For example, Oxygen outweighs Hydrogen 7.94:1 as components of water; 15.9:1 in hydrogen peroxide; Carbon outweighs Hydrogen 2.98:1 in methane; and outweighs Hydrogen 2.67:1 in carbon dioxide. These specifics are for illustrative purpose here: AFAIK H2O2 was not measured. Three things are notable in the numbers just presented:

(1) the numbers are constant -- water always decomposes into the same 7.94:1 ratio. (Ignore water deliberately taken from the depths of Lake Baikal!)
(2) the quotient (2.00) between two example results -- 15.9 for H2O2 and 7.94 for H2O -- leads directly to the conclusion that the O/H ratio is exactly twice in the former what it is in the latter.
(3) The ratios 7.94 : 1, 15.9 : 1, 2.98 : 1; 2.67 (or rather 8 ÷ 3) are all simple integers.

I think it was (1) by itself which pointed strongly to a theory of small molecules containing small numbers of a few atoms. If one rejects the atomic theory then what model would explain the constant ratios apparent in simple compounds?

The 2.00 quotient 15.9/7.94 in (2) leads directly to the idea that the chemical formula must be H_yO_2x and H_yO_x. How can you explain this without an atomic theory?

The result in (3) may have been invisible to the early 19th century chemists. The "error bars" on their weighings may have been too large to even notice this fact. (What were the experimental precisions and how did they improve over time?) That these ratios are simple integers depends on both nucleons having almost the same weight; and on the fact that the three afore-mentioned elements are nearly mono-isotopal -- O¹⁶ (99.8%), C¹² (98.9%), H¹(>99.9%). Early experiments also used some bi-isotopal atoms, e.g. Chlorine (Cl³⁵ 75%, Cl³⁷ 25%). But again, I don't know when results like (3) were actually first noted.

So my question: Do the results by Dalton et al strongly support the atomic theory? If not, what alternate theory was (or could have been) proposed at that time?

In fact skepticism about the atomic theory persisted until a 1905 paper written by Albert Einstein (who supplied math to supplement Lucretius' observation almost 2000 years earlier). Why did this disbelief in atomism persist?

Max Planck was one of the most notable opponents of atomism. He correctly noted that the atomic theory was incompatible with classical thermodynamics (in particular the Second Law). This problem is solved with statistical thermodynamics, introduced in the 1870's by Maxwell, Clausius and Boltzmann (decades before Einstein's paper on Brownian motion). These three luminaries preceded Planck (who got his PhD in 1879). Even today thermodynamics is often rendered in its classical form rather than the correct statistical view. (@ Experts - Do I write correctly?)

 

[bilby]

The original atomic theory just said that you cannot keep cutting something in half forever.

Once you get a small enough piece, it is un-cuttable (literally Atom, where 'tom' is the Greek 'to cut' - which is also the root of the word 'microtome'; I presume that on an atheist board I need not explain the meaning of the 'a-' prefix)

That's rather different from the later, post enlightenment, idea of chemical elements.

In Ancient Greek philosophy, the idea was that you could cut (eg) a block of wood only a finite number of times, and would end up with a (very small) block of wood that could not be cut.

Later, it was understood that wood was not a single substance, but rather a mixture of substances - cellulose, water, lignin, etc. which could be separated out by (al)chemical and/or physical processes. The idea then was that these "pure" substances could be cut only so small, but no smaller; This is, to modern science, a molecular theory rather than an atomic theory.

Then people started breaking down those substances, as described in the OP, and the modern "element" became a thing. You can 'cut' a molecule into its individual atoms, but those atoms are Oxygen or Hydrogen atoms, not water atoms (for example).

So the smallest "atom" of water is an H₂O molecule, and the smallest "atom" of Oxygen is the O₂ molecule - if you cut them any smaller, they are no longer the same substance, with the same physical and chemical properties as the larger particle with which you began.

But for whatever reason, we decided to use "atom" for the modern meaning - a nucleus with a number of protons which define its place in the periodic table of the elements - and to call the smallest possible subdivision of a given chemical substance "molecules". Of course, for the Noble Gases, the atom and the molecule are the same thing.

And we know that atoms are cuttable. We can seperate nucleii into protons and neutrons; And we can separate nucleons into quarks...

Where does it end? Have we reached the bottom of the hierarchy? Will someone 'split the quark' one day?

If we accept Quantum Field Theory (and I suspect we should), the whole question is moot, because there are no particles at all, just regions of high amplitude in universe-spanning fields.

Atomic theory is a set of different and mutually incompatible theories; What it is depends on when you ask, and who you ask. You can cut a Uranium atom in two, but it won't still be Uranium. OTOH, you can cut a Carbon Monoxide molecule in two, but it won't still be Carbon Monoxide, so is there an important difference in 'cutability' between these things?

Atom has lost its meaning of "uncuttable", and become a label for a particular class of entities somewhere in the middle of a hierarchy of the microscopic. The ancient Greek Philosophers wouldn't agree with our way of using the word, and likely nor would the nineteenth century chemists, who would concur with the Greeks that if you can break or cut it into pieces, it's not an atom.

So in a very real sense, the opponents of the atomic theory have been vindicated. There are no indivisible particles; But bizarrely that's because at sufficiently small scales, there are no particles at all.

 

[Swammerdami]

Atomic theory in the 19th-century sense is the claim that substances (elements) had a smallest form beyond which further division, if possible, might yield constituents but would NOT yield the substance itself. Note that multi-atom molecules also fit that 'smallest form' definition. Dissenters like Max Planck thought that substances scaled along a continuum where there were no single atoms or single molecules beyond which further division, if possible at all, would change the nature of the substance. In other words, the existence of elementary particles or even quarks would not contradict, but rather support the atomist side in the 19th century debate.

The notion of molecules each composed of a finite number of atoms seems to explain well the early results in quantitative chemistry. My question is What model would fit the non-atomist position?

The short paragraph in OP which mentioned thinkers in Greek Antiquity was a distraction. I just thought it fun to connect Einstein's conclusion from Brownian motion with that of Lucretius almost 2000 years earlier.