John Dalton

Quick Facts

John Dalton FRS (6 September 1766 – 27 July 1844) was an English chemist, physicist, and meteorologist. He is best known for his pioneering work in the development of modern atomic theory; and his research into colour blindness, sometimes referred to as Daltonism, in his honour.

Early Life

John Dalton was born into a Quaker family at the settlement of Eaglesfield, near the town of Cockermouth, in the county of Cumberland, England in 1766. His father was a weaver. He received his early education from his father and from Quaker John Fletcher, who ran a private school at Pardshaw Hall. With his family too poor to support him for long, he began to earn his living at the age of ten in the service of a wealthy local Quaker, Elihu Robinson. It is said he began teaching at a local school at age 12, and was proficient in Latin at age 14.

Early Careers

He joined his older brother Jonathan at age 15 in running a Quaker school at Stramongate in Kendal, about forty five miles from his home. Around age 23 Dalton may have considered studying law or medicine, but his relatives did not encourage him, perhaps because being a Dissenter (a Christian opposed to a state religion and mandatory membership in the Church of England), he was barred from attending English universities. He acquired much scientific knowledge from informal instruction by John Gough, a blind philosopher who was gifted in the sciences and arts. At age 27 he was appointed teacher of mathematics and natural philosophy at the "New College" in Manchester, a dissenting academy. He remained there until age 34, when the college's worsening financial situation led him to resign his post and begin a new career as a private tutor for mathematics and natural philosophy.

Atomic Theory

The most important of all Dalton's investigations are those concerned with the atomic theory in chemistry. While his name is inseparably associated with this theory, the origin of Dalton's atomic theory is not fully understood. It has been proposed that this theory was suggested to him either by researches on ethylene (olefiant gas) and methane (carburetted hydrogen) or by analysis of nitrous oxide (protoxide of azote) and nitrogen dioxide (deutoxide of azote), both views resting on the authority of Thomas Thomson. However, a study of Dalton's own laboratory notebooks, discovered in the rooms of the Lit & Phil,  concluded that so far from Dalton being led by his search for an explanation of the law of multiple proportions to the idea that chemical combination consists in the interaction of atoms of definite and characteristic weight, the idea of atoms arose in his mind as a purely physical concept, forced upon him by study of the physical properties of the atmosphere and other gases. The first published indications of this idea are to be found at the end of his paper on the absorption of gases already mentioned, which was read on 21 October 1803, though not published until 1805. Here he says:

Why does not water admit its bulk of every kind of gas alike? This question I have duly considered, and though I am not able to satisfy myself completely I am nearly persuaded that the circumstance depends on the weight and number of the ultimate particles of the several gases.

The main points of Dalton's atomic theory were:

1. Elements are made of extremely small particles called atoms.
2. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.
3. Atoms cannot be subdivided, created, or destroyed.
4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
5. In chemical reactions, atoms are combined, separated, or rearranged.

Dalton proposed an additional "rule of greatest simplicity" that created controversy, since it could not be independently confirmed.

When atoms combine in only one ratio, "..it must be presumed to be a binary one, unless some cause appear to the contrary".

For elements that combined in multiple ratios, their combinations were assumed to be the simplest ones possible. Two combinations resulted in a binary and a ternary compound. This was merely an assumption, derived from faith in the simplicity of nature. No evidence was then available to scientists to deduce how many atoms of each element combine to form compound molecules. But this or some other such rule was absolutely necessary to any incipient theory, since one needed an assumed molecular formula in order to calculate relative atomic weights. In any case, Dalton's "rule of greatest simplicity" caused him to assume that the formula for water was OH and ammonia was NH, quite different from our modern understanding (H₂O, NH₃).

Despite the uncertainty at the heart of Dalton's atomic theory, the principles of the theory survived. To be sure, the conviction that atoms cannot be subdivided, created, or destroyed into smaller particles when they are combined, separated, or rearranged in chemical reactions is inconsistent with the existence of nuclear fusion and nuclear fission, but such processes are nuclear reactions and not chemical reactions. In addition, the idea that all atoms of a given element are identical in their physical and chemical properties is not precisely true, as we now know that different isotopes of an element have slightly varying weights. However, Dalton had created a theory of immense power and importance. Indeed, Dalton's innovation was fully as important for the future of the science as Antoine Laurent Lavoisier's oxygen-based chemistry had been.

Atomic Weights

Dalton proceeded to print his first published table of relative atomic weights. Six elements appear in this table, namely hydrogen, oxygen, nitrogen, carbon, sulfur, and phosphorus, with the atom of hydrogen conventionally assumed to weigh 1. Dalton provided no indication in this first paper how he had arrived at these numbers. However, in his laboratory notebook under the date 6 September 1803 there appears a list in which he sets out the relative weights of the atoms of a number of elements, derived from analysis of water, ammonia, carbon dioxide, etc. by chemists of the time.

Various atoms and molecules as depicted in John Dalton's A New System of Chemical Philosophy (1808).

It appears, then, that confronted with the problem of calculating the relative diameter of the atoms of which, he was convinced, all gases were made, he used the results of chemical analysis. Assisted by the assumption that combination always takes place in the simplest possible way, he thus arrived at the idea that chemical combination takes place between particles of different weights, and it was this which differentiated his theory from the historic speculations of the Greeks, such as Democritus and Lucretius.

The extension of this idea to substances in general necessarily led him to the law of multiple proportions, and the comparison with experiment brilliantly confirmed his deduction. It may be noted that in a paper on the proportion of the gases or elastic fluids constituting the atmosphere, read by him in November 1802, the law of multiple proportions appears to be anticipated in the words: "The elements of oxygen may combine with a certain portion of nitrous gas or with twice that portion, but with no intermediate quantity", but there is reason to suspect that this sentence may have been added some time after the reading of the paper, which was not published until 1805.

Compounds were listed as binary, ternary, quaternary, etc. (molecules composed of two, three, four, etc. atoms) in the New System of Chemical Philosophy depending on the number of atoms a compound had in its simplest, empirical form.

He hypothesized the structure of compounds can be represented in whole number ratios. So, one atom of element X combining with one atom of element Y is a binary compound. Furthermore, one atom of element X combining with two elements of Y or vice versa, is a ternary compound. Many of the first compounds listed in the New System of Chemical Philosophy correspond to modern views, although many others do not.

Dalton used his own symbols to visually represent the atomic structure of compounds. These were depicted in theNew System of Chemical Philosophy, where Dalton listed twenty elements and seventeen simple molecules.

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