E.g. a quote from Justus von Liebig, 17th Chemical Letter, 1858, in German:

Wir können ein Stück Zucker, auch wenn wir es noch so fein reiben, nicht flüssig machen, noch viel weniger können wir durch eine mechanische Gewalt ein Zuckeratom zersetzen, ein Kohlenstoff oder Wasserstoffatom davon losreissen.
Wir können in einer Zuckerlösung durch Schütteln Zuckeratome und Wasseratome neben einander hin und her bewegen, aber die Elemente derselben wechseln damit ihren Platz nicht.

Literal translation:

We cannot make a piece of sugar liquid, no matter how finely we grind it, much less can we use mechanical force to decompose a sugar atom or tear away a carbon or hydrogen atom from it.
We can move sugar atoms and water atoms back and forth next to each other in a sugar solution by shaking, but the elements do not change their place.

How widespread was it to call those chemical entities "atoms", which are molecules or atoms in modern terminology?

Was this also common in the English literature?


1 Answer 1


Summary: I don't think there was a definite point when it changed meaning from "smallest component with the same chemistry" to uniquely "compound of atoms". It was mostly a gradual transition between 1860 and 1908.

Origin of the word

"Molecule" as a word comes from Latin is very ancient and was not necessarily used for science. According to Online Etymology Dictionary: Molecule it can be traced back to Réné Descartes. However the modern use of the word was given by Amadeus Avogadro in 1811 meaning "smallest part into which a substance can be divided without destroying its chemical character". Note than an atom is usually defined as the "smallest indivisible component" all atoms are molecules under older definitions (but not all molecules are atoms).

Success of molecular theory

In the 19th century the concept of atoms and molecules was not clear. At the beginning of the century John Dalton had just published his idea of atoms, and atoms could combine together to form compounds.

Note that during this period, some scientists were against the idea of molecules and atoms, so in an alternative theory a gas was considered to be composed of some indivisible fluid instead.

Avogadro, and independently André-Marie Ampère advanced the idea of "composite molecules". This hypothesis helped explain some inconsistencies with Gay-Lussac law of combining volumes and Dalton's theory. According to Wikipedia History of molecular theory even if Avogadro reinvented the word, he still used molecule in the old sense:

Avogadro uses the name "molecule" for both atoms and molecules. Specifically, he uses the name "elementary molecule" when referring to atoms and to complicate the matter also speaks of "compound molecules" and "composite molecules".

The success of Avogadro-Ampère hypothesis is possibly due to Stanislao Cannizzaro. Some experimental investigations using organic chemistry were in accordance with Avogadro's law however when conducting similar experiments with certain inorganic substances, deviations from the law seemed apparent. It was Cannizzaro, addressing the Karlsruhe Congress in 1860, resolved this seeming contradiction, based on the ideas of Ampère and Avogadro (the later had died a couple of years before). He clarified that these deviations were a result of molecular dissociations occurring at specific temperatures. The hypothesis became Avogadro's law and made possible the measurements of atomic weights to distinguish between atoms and compound atoms. However Cannizzaro still uses molecules in the original sense, see Sketch of a Course of Chemical Philosophy.. Similarly August Kekulé, the congress organizer, wrote in a note: "A molecule is 1, at most 2 atoms."

Evolution of the meaning

The evolution from molecule as smallest part and molecule as compound atom took some time. In an article in 1881 Popular Science Monthly "What is a molecule", the definition alternates between the two:

He "decomposes," or breaks up, these molecules into "atoms"

but also

and every molecule, though physically indivisible, can be broken up chemically into atoms, which are themselves the molecules

a similar mixed treatment is given by James Clerk Maxwell 1873 "Molecules".

In 1904, Walther Nersnt gives a similar mixed account but is already much closer to the modern meaning (in Theoretical Chemistry from the Standpoint of Avogardro's Rule & Thermodynamics):

Therefore these molecules must be divisible, and such a separation takes place when a compound decomposes into its elements. Thus we reach the assumption that a molecule does not continuously fill the space appropriated by it, but is a discrete aggregate of separable particles in its total space. These particles we call "atoms." These atoms, by the union of which, molecules are made, are all alike if they belong to the molecule of one element, but different if they belong to the molecule of a chemical compound. Only in the first case [i.e. of an element] can it happen that a molecule may consist of a single atom. The force which binds the atoms in molecular union, we call the "chemical affinity" of the atom.

It is often considered that the final demonstration of the existence of molecules was given by Jean Perrin in his 1908, with his work on Brownian motion. But in this work he clearly refers to water as a molecule and hydrogen, oxygen, potassium and other elements as atoms, so the transition to the modern meaning was already cleared up by this time. Interestingly enough Perrin refers to the electron as the atom of electricity.


As suggested by OP, already in 1911, Encyclopedia Brittanica reads "a compound atom, or as we now say a “molecule”.

Some of this text was based on: H.H. Kubbinga, "The first ‘molecular” theory (1620): Isaac Beeckman (1588–1637)" (1988)

Suggestion: If you wish to deep further into when the transition was made I suggest finding a German speaker, I think something could be found in the works of van 't Hoff, Boltzmann, Weber, Lorentz and even in Einstein's doctoral thesis.

More on the Karlsruhe Congress

After writing all this I have found that the question "What is a molecule and what is an atom" was one of the three main topics during the Karlsruhe Congress:

Shall a difference be made between the expressions “molecule” and “atom,” such that a molecule be named the smallest particle of bodies which can enter into chemical reactions and which may be compared to each other in regard to physical properties, atoms being the smallest particles of those bodies which are contained in molecules?

See also:

Mr. H. Kopp, summarizing the discussion, says that the need to separate the idea of the molecule from that of the atom appears to be established; that the notion of the molecule can be fixed with the help of purely chemical considerations; that the definition does not have to involve density alone; and, finally, that it appears natural to call the largest quantity the molecule, and the smallest quantity the atom. In concluding, the speaker formulates the first question to be put to the assembly. This question is as follows: "Is it appropriate to establish a distinction between the terms molecule and atom, and to call molecules, which are comparable as far as physical properties go, the smallest quantities of bodies which enter into or come out of a reaction, and to call atoms the smallest quantities of bodies which are contained in these molecules?"

  • $\begingroup$ In the 1911 Britannica we read "... form a compound atom, or as we now say a molecule". It's still difficult to see how much this is change in terminology and how much genuine scientific progress. $\endgroup$
    – viuser
    Jan 17 at 22:24

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