The theory of atoms goes back to ancient history – there is evidence in both Ancient India and Ancient Greece of a belief that matter was made up of tiny, indivisible parts. The word ‘atom’ comes from Ancient Greek, ‘a-’ meaning ‘not’ (as in ‘atheist’ - ‘a-theist’ – ‘not a theist’) and ‘-tom’ meaning ‘to cut’ (as in ‘tonsillecTOMy’ – ‘to cut out the tonsils’). The idea was that one could take a piece of wood, for example, and cut it into two parts, then into two again, until one reached a point where the pieces were so small, they could not be cut again. The theory goes back to the shadowy Leucippus, whose teachings are so intertwined with those of his pupil Democritus that some even doubt his existence.
|Democritus - the 'Laughing Philosopher'|
Democritus is reputed to have travelled throughout Asia Minor, India and Egypt, before returning to Abdera, where he had his school. His teachings were taken up, and elaborated on, by the Epicureans (especially by the Roman Lucretius, in his poem De Rarum Natura ‘On the Nature of Things’). All matter is made from atoms, which are indivisible, and immutable – they cannot be made and they cannot be destroyed. They come together and make material objects, and disseminate before passing on to make something else. They are eternal; they have always existed and always will exist.
Atomism fell out of favour in later years, as Aristotelian scholasticism came to the fore. In this view, things were made from ‘elements’ – fire, water, earth and air – and were mutable; they could be changed into other things (as in the alchemical view that base metals could be changed into gold). As empirical (from experiment and observation) chemistry began, some early doubts were aired; in his The Sceptical Chymist (1661) Robert Boyle wrote,
“I consider that if it be as true, as 'tis probable, that compounded bodies differ from one another but in the various textures resulting from the bigness, shape, motion, and contrivance of their small parts,”
although Boyle’s position tended more to corpuscularianism, wherein the ‘corpuscles’ could, in theory at least, be divided.
|Robert Boyle - The Sceptical Chymist - 1661|
Other scientists, (Higgins, Richter, Becher, Stahl et al), considered the possibility of atoms and their combination into compounds, but it was not until John Dalton’s work that we find a worked out theory of the atom. William Higgins, a nephew and pupil of Dr Bryan Higgins, published his Comparative View of Phlogistic and Antiphlogistic Theories (1789, 2nd ed. 1791), which he was later to claim pre-dated Dalton, but his assertions were quite probably prompted by his friend and mentor Humphrey Davy, who opposed Dalton’s theory. Higgins cried plagiarism in 1814, but his earlier works are a confused jumble of arbitrary speculations and his claim does not stand up. It has been said of Higgins “He had seen the right road, but dared not go farther.” There are also contemporary evidences that Dalton had not read Higgins in any case, (he was not known for his wide reading, preferring his own experiments instead).
|Dalton's Atomic Symbols|
From his early writings, it is evident that Dalton believed that gases were made up of ‘particles’, which combined in different proportions and compositions to produce different gases. It was an investigation into why different elements were absorbed into water in different amounts that led him to consider that this could be due to the weight of the ‘particles’ in those elements. Dalton began to present his findings in a series of papers to the Literary and Philosophical Society of Manchester from 1801 onwards, and an outline of his atomic theory was first presented, by consent, in Thomas Thomson’s System of Chemistry (1807) followed in the next year by Dalton’s own New System of Chemical Philosophy.
|John Dalton - A System of Chemical Philosophy - 1808|
Dalton used a system of representing the various elements that used circular symbols, used in various combinations to correspond to the compounds described. It is not a very practical way of doing so, as one is forced to constantly count the various symbols, although it does have a certain pictorial beauty.
If you look at the fourth molecule down in Dalton’s list, you will see he (incorrectly) assigns one atom each of hydrogen and oxygen to water (he thought a simple binary combination would be the most elegant form of a molecule; his rule of ‘greatest simplicity’). His system was superseded by the more familiar form of chemical formula notation derived by Berzelius in 1815, but Dalton was ‘horrified’ by this ‘chaos of atoms’.
Dalton’s other laws of atomic theory are; a) Elements are made of extremely small particles called atoms, b) Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties, c) Atoms cannot be subdivided, created, or destroyed, d) Atoms of different elements combine in simple whole-number ratios to form chemical compounds, and e) In chemical reactions, atoms are combined, separated, or rearranged.
|The now-superseded 'planetary' representation of an atom|
We now know that atoms are composed of smaller parts – electrons, protons and neutrons, and that the protons and neutrons that make up the nucleus are composed of the even smaller quarks - however, splitting an atom is a nuclear process, not a chemical one. But we are getting into the realms of quantum physics now, and I am uncertain of the probability of our return. So let’s leave it here.