Editing History of atomic theory (section)

=={{anchor|Dalton}}Dalton's law of multiple proportions==

[[File:Daltons symbols.gif|thumb|right|From ''A New System of Chemical Philosophy'' (John Dalton 1808).]]
[[John Dalton]] studied data gathered by himself and by other scientists. He noticed a pattern that later came to be known as the [[law of multiple proportions]]: in compounds which contain two particular elements, the amount of Element A per measure of Element B will differ across these compounds by ratios of small whole numbers. This suggested that each element combines with other elements in multiples of a basic quantity.<ref name="Pullman 1998 p. 199">Pullman (1998). ''The Atom in the History of Human Thought'', p. 199: "The constant ratios, expressible in terms of integers, of the weights of the constituents in composite bodies could be construed as evidence on a macroscopic scale of interactions at the microscopic level between basic units with fixed weights. For Dalton, this agreement strongly suggested a corpuscular structure of matter, even though it did not constitute definite proof."</ref>

In 1804, Dalton explained his atomic theory to his friend and fellow chemist [[Thomas Thomson (chemist)|Thomas Thomson]], who published an explanation of Dalton's theory in his book ''A System of Chemistry'' in 1807. According to Thomson, Dalton's idea first occurred to him when experimenting with "olefiant gas" ([[ethylene]]) and "carburetted hydrogen gas" ([[methane]]). Dalton found that "carburetted hydrogen gas" contains twice as much hydrogen per measure of carbon as "olefiant gas", and concluded that a molecule of "olefiant gas" is one carbon atom and one hydrogen atom, and a molecule of "carburetted hydrogen gas" is one carbon atom and two hydrogen atoms.<ref>Thomas Thomson (1831). ''A History of Chemistry, Volume 2''. p. 291</ref> In reality, an [[ethylene]] molecule has two carbon atoms and four hydrogen atoms (C<sub>2</sub>H<sub>4</sub>), and a [[methane]] molecule has one carbon atom and four hydrogen atoms (CH<sub>4</sub>). In this particular case, Dalton was mistaken about the formulas of these compounds, but he got them right in the following examples:

'''''Example 1 — tin oxides:''''' Dalton identified two types of [[tin oxide (disambiguation)|tin oxide]]. One is a grey powder that Dalton referred to as "the protoxide of tin", which is 88.1% tin and 11.9% oxygen. The other is a white powder which Dalton referred to as "the deutoxide of tin", which is 78.7% tin and 21.3% oxygen. Adjusting these figures, in the grey powder there is about 13.5&nbsp;g of oxygen for every 100&nbsp;g of tin, and in the white powder there is about 27&nbsp;g of oxygen for every 100&nbsp;g of tin. 13.5 and 27 form a ratio of 1:2. These compounds are known today as [[tin(II) oxide]] (SnO) and [[tin(IV) oxide]] (SnO<sub>2</sub>).<ref>[[#refDalton1817|Dalton (1817). ''A New System of Chemical Philosophy'' vol. 2, p. 36]]</ref><ref>[[#refMelsen1952|Melsen (1952). ''From Atomos to Atom''. p. 137]]</ref> In Dalton's terminology, a "protoxide" is a molecule containing a single oxygen atom, and a "deutoxide" molecule has two. The modern equivalents of his terms would be ''monoxide'' and ''dioxide''.<ref>''Hawley's Condensed Chemical Dictionary'' 16th edition,  p. 1270</ref><ref>William Rossiter (1879). ''An Illustrated Dictionary of Scientific Terms'', p. 98</ref>

'''''Example 2 — iron oxides:''''' Dalton identified two oxides of iron. There is one type of iron oxide that is a black powder which Dalton referred to as "the protoxide of iron", which is 78.1% iron and 21.9% oxygen. The other iron oxide is a red powder, which Dalton referred to as "the intermediate or red oxide of iron" which is 70.4% iron and 29.6% oxygen. Adjusting these figures, in the black powder there is about 28&nbsp;g of oxygen for every 100&nbsp;g of iron, and in the red powder there is about 42&nbsp;g of oxygen for every 100&nbsp;g of iron. 28 and 42 form a ratio of 2:3. These compounds are [[iron(II) oxide]] and [[iron(III) oxide]] and their formulas are FeO and Fe<sub>2</sub>O<sub>3</sub> respectively. Iron(II) oxide's formula is normally written as FeO, but since it is a crystalline substance one could alternately write it as Fe<sub>2</sub>O<sub>2</sub>, and when we contrast that with Fe<sub>2</sub>O<sub>3</sub>, the 2:3 ratio stands out plainly. Dalton described the "intermediate oxide" as being "2 atoms protoxide and 1 of oxygen", which adds up to two atoms of iron and three of oxygen. That averages to one and a half atoms of oxygen for every iron atom, putting it midway between a "protoxide" and a "deutoxide".<ref>Dalton (1817). ''A New System of Chemical Philosophy'' vol. 2. pp. 28-34: "the intermediate or red oxide is 2 atoms protoxide and 1 of oxygen"</ref><ref>[[#refMillington1906|Millington (1906). ''John Dalton'', p. 113]]</ref>

'''''Example 3 — nitrogen oxides:''''' Dalton was aware of three oxides of nitrogen: "nitrous oxide", "nitrous gas", and "nitric acid".<ref>[[#refDalton1808|Dalton (1808). ''A New System of Chemical Philosophy'' vol. 1, pp. 316–319]]</ref> These compounds are known today as [[nitrous oxide]], [[nitric oxide]], and [[nitrogen dioxide]] respectively. "Nitrous oxide" is 63.3% nitrogen and 36.7% oxygen, which means it has 80 g of oxygen for every 140 g of nitrogen. "Nitrous gas" is 44.05% nitrogen and 55.95% oxygen, which means there is 160 g of oxygen for every 140 g of nitrogen. "Nitric acid" is 29.5% nitrogen and 70.5% oxygen, which means it has 320 g of oxygen for every 140 g of nitrogen. 80&nbsp;g, 160&nbsp;g, and 320&nbsp;g form a ratio of 1:2:4. The formulas for these compounds are N<sub>2</sub>O, NO, and NO<sub>2</sub>.<ref>[[#refDalton1808|Dalton (1808). ''A New System of Chemical Philosophy'' vol. 1. pp. 316–319]]</ref><ref>[[#refHolbrowEtAl2010|Holbrow et al. (2010). ''Modern Introductory Physics'', pp. 65–66]]</ref>

Dalton defined an atom as being the "ultimate particle" of a chemical substance, and he used the term "compound atom" to refer to "ultimate particles" which contain two or more elements. This is inconsistent with the modern definition, wherein an atom is the basic particle of a [[chemical element]] and a molecule is an agglomeration of atoms. The term "compound atom" was confusing to some of Dalton's contemporaries as the word "atom" implies indivisibility, but he responded that if a [[carbon dioxide]] "atom" is divided, it ceases to be carbon dioxide. The carbon dioxide "atom" is indivisible in the sense that it cannot be divided into smaller carbon dioxide particles.<ref name="Pullman 1998 p. 201"/><ref>Dalton, quoted in Freund (1904). ''The Study of Chemical Composition''. p. 288: "I have chosen the word atom to signify these ultimate particles in preference to particle, molecule, or any other diminiutive term, because I conceive it is much more expressive; it includes in itself the notion of indivisible, which the other terms do not. It may, perhaps, be said that I extend the application of it too far when I speak of compound atoms; for instance, I call an ultimate particle of carbonic acid a compound atom. Now, though this atom may be divided, yet it ceases to become carbonic acid, being resolved by such division into charcoal and oxygen. Hence I conceive there is no inconsistency in speaking of compound atoms and that my meaning cannot be misunderstood."</ref>

Dalton made the following assumptions on how "elementary atoms" combined to form "compound atoms" (what we today refer to as [[molecule]]s). When two elements can only form one compound, he assumed it was one atom of each, which he called a "binary compound". If two elements can form two compounds, the first compound is a binary compound and the second is a "ternary compound" consisting of one atom of the first element and two of the second. If two elements can form three compounds between them, then the third compound is a "quaternary" compound containing one atom of the first element and three of the second.<ref>[[#refDalton1817|Dalton (1817). ''A New System of Chemical Philosophy'' vol. 1, pp. 213–214]]</ref> Dalton thought that water was a "binary compound", i.e. one hydrogen atom and one oxygen atom. Dalton did not know that in their natural gaseous state, the ultimate particles of oxygen, nitrogen, and hydrogen exist in pairs (O<sub>2</sub>, N<sub>2</sub>, and H<sub>2</sub>). Nor was he aware of valencies. These properties of atoms were discovered later in the 19th century.{{citation needed|date=October 2024}}

Because atoms were too small to be directly weighed using the methods of the 19th century, Dalton instead expressed the weights of the myriad atoms as multiples of the hydrogen atom's weight, which Dalton knew was the lightest element. By his measurements, 7 grams of oxygen will combine with 1 gram of hydrogen to make 8 grams of water with nothing left over, and assuming a water molecule to be one oxygen atom and one hydrogen atom, he concluded that oxygen's atomic weight is 7. In reality it is 16. Aside from the crudity of early 19th century measurement tools, the main reason for this error was that Dalton didn't know that the water molecule in fact has two hydrogen atoms, not one. Had he known, he would have doubled his estimate to a more accurate 14. This error was corrected in 1811 by [[Amedeo Avogadro]]. Avogadro proposed that equal volumes of any two gases, at equal temperature and pressure, contain equal numbers of molecules (in other words, the mass of a gas's particles does not affect the volume that it occupies).<ref name="avogadro">{{cite journal|author=Avogadro, Amedeo|url=http://web.lemoyne.edu/~giunta/avogadro.html |title=Essay on a Manner of Determining the Relative Masses of the Elementary Molecules of Bodies, and the Proportions in Which They Enter into These Compounds|year=1811 |journal=Journal de Physique|volume=73|pages=58–76}}</ref> Avogadro's hypothesis, now usually called [[Avogadro's law]], provided a method for deducing the relative weights of the molecules of gaseous elements, for if the hypothesis is correct relative gas densities directly indicate the relative weights of the particles that compose the gases. This way of thinking led directly to a second hypothesis: the particles of certain elemental gases were pairs of atoms, and when reacting chemically these molecules often split in two. For instance, the fact that two liters of hydrogen will react with just one liter of oxygen to produce two liters of water vapor (at constant pressure and temperature) suggested that a single oxygen molecule splits in two in order to form two molecules of water. The formula of water is H<sub>2</sub>O, not HO. Avogadro measured oxygen's atomic weight to be 15.074.<ref>{{cite journal | first = Amedeo | last = Avogadro | author-link = Amedeo Avogadro | title = Essai d'une manière de déterminer les masses relatives des molécules élémentaires des corps, et les proportions selon lesquelles elles entrent dans ces combinaisons | journal = Journal de Physique | year = 1811 | volume = 73 | pages = 58–76 |url = https://books.google.com/books?id=MxgTAAAAQAAJ&pg=PA58 }} [http://web.lemoyne.edu/~giunta/avogadro.html English translation]</ref>