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.

 

[lpetrich]

Something that I've tried to find is what empirical evidence was ever offered for pre-Dalton atomism. I don't recall ever finding any.

Here's what I mean by empirical evidence: the  Law of definite proportions discovered in 1797. It states that some mixtures come in definite proportions while others do not. Rusting of iron follows definite proportions while mixing of salt and sugar in water doesn't.

John Dalton used this law to estimate the relative masses of different elements' atoms, first publishing in 1805.

 

[Swammerdami]

Antoine Lavoisier decomposed water into two elements in definite proportions. Supposedly he decomposed and described other compounds also, but I can't find concrete examples. archive.org has an English translation of Lavoisier's (most famous?) book, but I couldn't easily find useful examples there either.

I'm more interested in understanding why people did NOT accept the atomic theory even after Dalton's work. I found an article (excerpted below) which references a debate from 1869, but there were still better-informed objections circa 1900. (The greatest 19th century physicists, Maxwell, Boltzmann, Rutherford, Thomson, DID believe in atomism).

I mentioned upthread that Planck found the notion of finite (mechanical) atoms to conflict with classical thermodynamics. There's at least one other intelligent-seeming object to atomism made by a top physicist of the late 19th century, but what it was slips my mind! :whack:

Here's the article I mentioned. I think you can create a free account to read it without submitting any credit card. Many of the opponents of Dalton's theory accepted the idea of atoms, but thought Dalton had the wrong details.

Dalton remarked, and Berzelius agreed with him, that without an atomic
theory, definite proportions would be " mysterious," but this is not very
different from what Newton's Cartesian critics had said about gravity.
Against the help which some chemists derived from the atomic model, we
must set the alarm of " some timid persons" who suspected that the
atheistic system of the Greek atomists was being reintroduced. Although
most chemists of this epoch used the atomic theory in a rather attenuated
form, Prout took up an extreme position. Like Davy, he believed that the
elements were complex, but he regarded the atomic theory as no more than
a useful fiction. In his Gulstonian lectures of 1831, he declared that " the
light in which I have always been accustomed to consider it, has been very
analogous to that in which I believe most botanists now consider the
Linnaean system; namely, as a conventional artifice, exceedingly convenient
for many purposes, but which does not represent nature."
. . .

In using the atomic language and atomic ideas, it seems to me of great
importance that we should limit our words as much as possible to statements
of facts, and put aside into the realm of imagination all that is not in evi-
dence. Thus the question whether our elementary atoms are in their nature
indivisible, or whether they are built up of smaller particles, is one upon
which I, as a chemist, have no hold whatever, and I may say that in chemistry
the question is not raised by any evidence whatsoever.

 

[lpetrich]

The  Law of definite proportions was discovered by Joseph Proust in 1797, after the French revolutionaries executed Antoine-Laurent de Lavoisier in 1794 on bogus charges. From  Antoine Lavoisier

Lavoisier's importance to science was expressed by Lagrange who lamented the beheading by saying: "Il ne leur a fallu qu'un moment pour faire tomber cette tête, et cent années peut-être ne suffiront pas pour en reproduire une semblable." ("It took them only an instant to cut off this head, and one hundred years might not suffice to reproduce its like.")

However, Lavoisier stated the present-day conception of chemical elements, and most of his listed ones are either still recognized as elements or else oxides of now-recognized elements. The two exceptions are light and heat. His list: Lavoisier's List of Elements

Light, heat, O, N, H, S, P, C, Cl, F, B, Sb, As, Bi, Co, Cu, Au, Fe, Pb, Mn, Hg, Mo, Ni, Pt, Ag, Sn, W, Zn, CaO, MgO, BaO, Al2O3, SiO2

 

[lpetrich]

I have done a lot of searching for claimed empirical evidence of pre-Dalton atomism, but I have completely failed to discover any.

I say pre-Dalton atomism, because John Dalton stated an empirical case for atoms, and from my researches, the first one ever. He used the law of definite proportions and the hypothesis of simple numerical proportions to come up with estimates of relative masses of different elements' atoms: atomic weights. Some of his numbers were off, like for hydrogen relative to oxygen. He thought that water was HO rather than H2O.

BTW, before the statement of the law of definite proportions, there was no clear statement of the difference between a chemical compound and a mixture. That law was essentially that some mixtures follow definite proportions and some mixtures don't.

 

[lpetrich]

Atoms, in the broad sense, are indivisible units. That makes a molecule a kind of atom, and also atomic nuclei and elementary particles.

One describes elementary particles with quantum field theory, and that can be visualized as waves with fixed total amounts.

What	Disintegration Energy
Molecules	~ 10^(-9)
Atoms (strict sense)	~ 10^(-8)
Nuclei	~ 10^(-3)
Hadrons	~ 1
Electrons	> 10^5
Up and down quarks	> 10^6

The disintegration energy is the minimum relative energy that will cause disintegration, at least partial disintegration. Relative meaning relative to rest-mass energy.

The electron limit is from LEP experiments and the up and down quark limit from LHC experiments. Note that their limits are far above their rest masses, meaning extreme cancellation if they are bound states. So have we found "true atoms"?

 

[Loren Pechtel]

Atoms, in the broad sense, are indivisible units. That makes a molecule a kind of atom, and also atomic nuclei and elementary particles.

One describes elementary particles with quantum field theory, and that can be visualized as waves with fixed total amounts.

What	Disintegration Energy
Molecules	~ 10^(-9)
Atoms (strict sense)	~ 10^(-8)
Nuclei	~ 10^(-3)
Hadrons	~ 1
Electrons	> 10^5
Up and down quarks	> 10^6

The disintegration energy is the minimum relative energy that will cause disintegration, at least partial disintegration. Relative meaning relative to rest-mass energy.

The electron limit is from LEP experiments and the up and down quark limit from LHC experiments. Note that their limits are far above their rest masses, meaning extreme cancellation if they are bound states. So have we found "true atoms"?

I think anything with an energy to destroy well above it's rest mass is fundamental. What does it even mean to "destroy" when you are playing with energies quite sufficient for pair production?

 

[lpetrich]

The disintegration energy is the minimum relative energy that will cause disintegration, at least partial disintegration. Relative meaning relative to rest-mass energy.

The electron limit is from LEP experiments and the up and down quark limit from LHC experiments. Note that their limits are far above their rest masses, meaning extreme cancellation if they are bound states. So have we found "true atoms"?

I think anything with an energy to destroy well above it's rest mass is fundamental. What does it even mean to "destroy" when you are playing with energies quite sufficient for pair production?

"Destroy" in the sense of breaking apart a composite entity, not particle-antiparticle reactions.

If electrons and up and down quarks are composite, that would require very extreme cancellation between the constituents' rest masses and their binding energy, at least 1 part per million for these particles.

 

[lpetrich]

Some premodern atomists believed in kinds of atoms that we might find odd: fire atoms and soul atoms - The Problem of the Immortality of the Soul in Epicurus where "soul" is what makes living things alive. So this was vitalist atomism.

 

[Loren Pechtel]

The disintegration energy is the minimum relative energy that will cause disintegration, at least partial disintegration. Relative meaning relative to rest-mass energy.

The electron limit is from LEP experiments and the up and down quark limit from LHC experiments. Note that their limits are far above their rest masses, meaning extreme cancellation if they are bound states. So have we found "true atoms"?

I think anything with an energy to destroy well above it's rest mass is fundamental. What does it even mean to "destroy" when you are playing with energies quite sufficient for pair production?

"Destroy" in the sense of breaking apart a composite entity, not particle-antiparticle reactions.

If electrons and up and down quarks are composite, that would require very extreme cancellation between the constituents' rest masses and their binding energy, at least 1 part per million for these particles.

But how do you determine that you actually broke it apart as opposed to pair production or the like?

 

[lpetrich]

But how do you determine that you actually broke it apart as opposed to pair production or the like?

Particle Data Group - compendium of known elementary particles, their properties, and their known decays.

In "Summary Tables" is "Searches not in other sections (SUSY, compositeness, ...)" with "Scale Limits Λ for Contact Interactions (the lowest dimensional interactions with four fermions)", "Excited Leptons", and "Color Sextet and Octet Particles".

In "Reviews, Tables, Plots" is "Hypothetical Particles and Concepts" is "Quark and lepton compositeness, searches for (rev.)" with "Limits on contact interactions", "Limits on excited fermions".

So it's certain things that don't fit the Standard Model. An interaction can produce four fermions by  Bremsstrahlung (German: "braking radiation") and pair production, and one can calculate how much from the Standard Model. So is there any more than what the Standard Model predicts.

For example, quarks collide in the LHC, and as they approach each other, each one radiates an energetic photon. But that photon stays virtual, soon becoming a fermion-antifermion pair. Thus, four fermions.

I'm not linking to these pages, because they are all the 2024 edition.

 

[lpetrich]

Looking back into the history of post-Dalton atomism,  Valence (chemistry) is the number of chemical bonds an atom can have. Hydrogen 1, oxygen 2, nitrogen 3, carbon 4, ... though it becomes complicated for some elements. An atom can have double or triple bonds with another atom. It was proposed in the middle of the 19th cy., well after John Dalton's work.

The  Kinetic theory of gases was proposed in the 18th cy. as resulting from the motions of small particles. It was further elaborated in the late 19th cy., along with a theory of it,  Statistical mechanics. In the middle of the 18th cy., Mikhail Lomonosov explained that this particle theory accounts for  Diffusion in gases and liquids, and also in solids, and related effects:  Solvation (being dissolved) and  Extraction (chemistry).

So diffusion was pre-Dalton evidence of atoms.

But diffusion could have been observed by the ancient atomists, and I don't know of any of them who claimed that as evidence of atoms.