Editing Atom (section)

=== Dalton's law of multiple proportions ===
[[File:Daltons symbols.gif|thumb|right|Various atoms and molecules from ''A New System of Chemical Philosophy'' (John Dalton 1808).]]
In the early 1800s, John Dalton compiled experimental data gathered by him and other scientists and discovered a pattern now known as the "[[law of multiple proportions]]". He noticed that in any group of chemical compounds which all contain two particular chemical elements, the amount of Element A per measure of Element B will differ across these compounds by ratios of small [[whole numbers]]. This pattern suggested that each element combines with other elements in multiples of a basic unit of weight, with each element having a unit of unique weight. Dalton decided to call these units "atoms".<ref>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>

For example, there are two types of [[tin oxide (disambiguation)|tin oxide]]: one is a grey powder that is 88.1% tin and 11.9% [[oxygen]], and the other is a white powder that 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. Dalton concluded that in the grey oxide there is one atom of oxygen for every atom of tin, and in the white oxide there are two atoms of oxygen for every atom of tin ([[tin(II) oxide|SnO]] and [[tin dioxide|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>

Dalton also analyzed [[iron oxide]]s. There is one type of iron oxide that is a black powder which is 78.1% iron and 21.9% oxygen; and there is another iron oxide that is a red powder 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. Dalton concluded that in these oxides, for every two atoms of iron, there are two or three atoms of oxygen respectively. These substances are known today as [[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 we 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 for the oxygen is plain to see.<ref>[[#refDalton1817|Dalton (1817). ''A New System of Chemical Philosophy'' vol. 2, p. 28]]</ref><ref>[[#refMillington1906|Millington (1906). ''John Dalton'', p. 113]]</ref>

As a final example: [[nitrous oxide]] is 63.3% [[nitrogen]] and 36.7% oxygen, [[nitric oxide]] is 44.05% nitrogen and 55.95% oxygen, and [[nitrogen dioxide]] is 29.5% nitrogen and 70.5% oxygen. Adjusting these figures, in nitrous oxide there is 80&nbsp;g of oxygen for every 140&nbsp;g of nitrogen, in nitric oxide there is about 160&nbsp;g of oxygen for every 140&nbsp;g of nitrogen, and in nitrogen dioxide there is 320&nbsp;g of oxygen for every 140&nbsp;g of nitrogen. 80, 160, and 320 form a ratio of 1:2:4. The respective formulas for these oxides are [[nitrous oxide|N<sub>2</sub>O]], [[nitric oxide|NO]], and [[nitrogen dioxide|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>