ABSTRACT
At the time of writing, the existence of molecules has been viewed as proven for about a century. Although various philosophers had argued that matter must consist of small, indivisible particles for millennia, early scientific evidence for molecules came around the early 19th century after chemists noticed that, in order to form chemical compounds, their components or “elements” have to be mixed in certain simple proportions. For example, carbon was known to form two different oxides. To make the first one out of one gram of carbon, it would have to be combined with 1.333 grams of oxygen. In contrast, the second oxide would require 2.666 grams of oxygen. John Dalton explained such observations by positing that chemical compounds consist of molecules, which, in turn, are formed from atoms. The molecule of the first oxide (CO, in modern notation) consists of one carbon atom and one oxygen atom, while the second one (CO2) has one carbon and two oxygen atoms, which immediately explains why it requires twice as much oxygen. Furthermore, one readily concludes that the ratio of the mass of the oxygen and the carbon atom must be equal to 1.333 ≈ 4/3. By analyzing such proportions in various chemical compounds, an internally consistent list of relative atomic masses was established. For example, if the hydrogen mass is taken as the atomic mass unit, then the masses of carbon, nitrogen, and oxygen are, respectively, equal to 12, 14, and 16. Unfortunately, the absolute mass of an atom cannot be determined in this way. Further refined by his followers, notably by Amedeo Avogadro who proposed the existence of diatomic molecules such as O2 or H2, Dalton’s ideas became widely accepted as the 19th century progressed. Despite overwhelming indirect evidence for Dalton’s atomic theory, however, the lack of any direct observations of atoms and molecules led many prominent scientists to view molecules as convenient mathematical devices rather than real physical entities.
