Editing History of atomic theory (section)

==Opposition to atomic theory==
Dalton's atomic theory attracted widespread interest but not everyone accepted it at first. The law of multiple proportions was shown not to be a universal law when it came to organic substances, whose molecules can be quite large. For instance, in [[oleic acid]] there is 34&nbsp;g of hydrogen for every 216&nbsp;g of carbon, and in [[methane]] there is 72&nbsp;g of hydrogen for every 216&nbsp;g of carbon. 34 and 72 form a ratio of 17:36, which is not a ratio of small whole numbers. We know now that carbon-based substances can have very large molecules, larger than any the other elements can form. Oleic acid's formula is C<sub>18</sub>H<sub>34</sub>O<sub>2</sub> and methane's is CH<sub>4</sub>.<ref>[[#refTrusted1999|Trusted (1999). ''The Mystery of Matter'', p. 73]]</ref> The law of multiple proportions by itself was not complete proof, and atomic theory was not universally accepted until the end of the 19th century.<ref name="Pullman 1998 p. 199"/>

One problem was the lack of uniform nomenclature. The word "atom" implied indivisibility, but Dalton defined an atom as being the ultimate particle of any [[chemical substance]], not just the elements or even matter per se. This meant that "compound atoms" such as carbon dioxide could be divided, as opposed to "elementary atoms". Dalton disliked the word "molecule", regarding it as "diminutive".<ref name="Pullman 1998 p. 201"/><ref>Freund (1904). ''The Study of Chemical Composition''. p. 288</ref> [[Amedeo Avogadro]] did the opposite: he exclusively used the word "molecule" in his writings, eschewing the word "atom", instead using the term "elementary molecule".<ref>[[#refPullman1998|Pullman (1998). ''The Atom in the History of Human Thought'', p. 202]]</ref> [[Jöns Jacob Berzelius]] used the term "organic atoms" to refer to particles containing three or more elements, because he thought this only existed in organic compounds. [[Jean-Baptiste Dumas]] used the terms "physical atoms" and "chemical atoms"; a "physical atom" was a particle that cannot be divided by physical means such as temperature and pressure, and a "chemical atom" was a particle that could not be divided by chemical reactions.<ref>Jean-Baptiste Dumas (1836). ''Leçons sur la philosophie chimique'' [''Lessons on Chemical Philosophy'']. 285–287</ref>

The modern definitions of ''atom'' and ''molecule''—an atom being the basic particle of an element, and a molecule being an agglomeration of atoms—were established in the late half of the 19th century. A key event was the [[Karlsruhe Congress]] in Germany in 1860. As the first international congress of chemists, its goal was to establish some standards in the community. A major proponent of the modern distinction between atoms and molecules was [[Stanislao Cannizzaro]].

{{blockquote|The various quantities of a particular element involved in the constitution of different molecules are integral multiples of a fundamental quantity that always manifests itself as an indivisible entity and which must properly be named atom.|[[Stanislao Cannizzaro]], 1860<ref>Pullman (1998). ''The Atom in the History of Human Thought''. p. 207</ref>}}

Cannizzaro criticized past chemists such as [[Jöns Jacob Berzelius|Berzelius]] for not accepting that the particles of certain gaseous elements are actually pairs of atoms, which led to mistakes in their formulation of certain compounds. Berzelius believed that hydrogen gas and chlorine gas particles are solitary atoms. But he observed that when one liter of hydrogen reacts with one liter of chlorine, they form two liters of [[hydrogen chloride]] instead of one. Berzelius decided that Avogadro's law does not apply to compounds. Cannizzaro preached that if scientists just accepted the existence of single-element molecules, such discrepancies in their findings would be easily resolved. But Berzelius did not even have a word for that. Berzelius used the term "elementary atom" for a gas particle which contained just one element and "compound atom" for particles which contained two or more elements, but there was nothing to distinguish H<sub>2</sub> from H since Berzelius did not believe in H<sub>2</sub>. So Cannizzaro called for a redefinition so that scientists could understand that a hydrogen ''molecule'' can split into two hydrogen ''atoms'' in the course of a chemical reaction.<ref>[https://archive.org/details/sketchofcourseof00cannrich/page/2/mode/2up Cannizzaro (1858). ''Sketch of a Course of Chemical Philosophy''. pp. 2–4]</ref>

A second objection to atomic theory was philosophical. Scientists in the 19th century had no way of directly observing atoms. They inferred the existence of atoms through indirect observations, such as Dalton's law of multiple proportions. Some scientists adopted positions aligned with the philosophy of [[positivism]], arguing that scientists should not attempt to deduce the deeper reality of the universe, but only systemize what patterns they could directly observe.<ref name=Pullman-1998/>{{rp|232}}

This generation of anti-atomists can be grouped in two camps. 
The "equivalentists", like [[Marcellin Berthelot]], believed the theory of [[equivalent weight]]s was adequate for scientific purposes. This generalization of Proust's law of definite proportions summarized observations.  For example, 1 gram of hydrogen will combine with 8 grams of oxygen to form 9 grams of water, therefore the "equivalent weight" of oxygen is 8 grams. The "energeticist", like [[Ernst Mach]] and [[Wilhelm Ostwald]], were philosophically opposed to hypothesis about reality altogether. In their view, only energy as part of thermodynamics should be the basis of physical models.<ref name=Pullman-1998/>{{rp|237}}

These positions were eventually quashed by two important advancements that happened later in the 19th century: the development of the [[periodic table]] and the discovery that molecules have an internal architecture that determines their properties.<ref>[[#refPullman1998|Pullman (1998). ''The Atom in the History of Human Thought'', p. 226]]: "The first development is the establishment of the periodic classification of the elements, marking the successful climax of concerted efforts to arrange the chemical properties of elements according to their atomic weight. The second is the emergence of structural chemistry, which ousted what was a simple and primitive verbal description of the elemental composition, be it atomic or equivalentist, of substances and replaced it with a systematic determination of their internal architecture."</ref>