Atoms
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If you take an apple and cut it in half, and then cut one of the halves in two, and then the quarter, and keep doing this, will you ever come to a point where you cannot divide the apple further? in the 5th century BCE Democritus argued that there is a fundamental entity that cannot be divided, and that all matter is made of these indivisible basic entities, that he called atoms (from the Greek atomos). The actual existence of atoms was disputed until late into the nineteenth century, but certain discoveries were being made. Experiments by Robert Boyle, Jacques Charles and Louis Guy-Lussac had provided accurate descriptions of the relations between the volume, pressure and temperature of gasses, but no fundamental theory of the processes involved was known. It was Maxwell (the same James Clerk Maxwell we met earlier), building on earlier work by Bernoulli, Rumford and Joule, who successfully presented a mathematical foundation for the kinetic theory of gasses. This work was further advanced by Ludwig Boltzmann. In the theory, the properties of gasses are elegantly described in terms of the thermal motion of tiny molecules that make up the gasses. This molecular hypothesis assumed the following: 1. The thermal properties of gasses (and, by extension, liquids and solids) is a result of the kinetic energy of the random motions of the molecules that make up the gas. 2. Newton's laws of motion may be used in the analysis of these motions. 3. All interactions between molecules are perfectly elastic - i.e. there is no friction on the molecular scale. 4. The motions of the molecules (because of their small individual masses) are virtually unaffected by gravitational forces, and the only intermolecular forces involved are those due to actual collisions (with each other or with the walls of their container). 5. The molecules are so small that their own volume is completely negligible when compared with the container that holds them (but they still are large enough to collide with each other). As the kinetic theory built upon these assumptions successfully portrayed the behavior of gasses, including accurate predictions of the laws of Charles, Boyle and Guy-Lussac, the molecular structure of matter was undeniable.

Chemistry

As the kinetic theory showed that the behavior of gasses could be explained in terms of the interaction of tiny particles (molecules), others were making advances in understanding chemical interactions. Lavoisier had shown (ca. 1780) that conservation of mass was maintained in chemical reactions (i.e. the product of a chemical reaction had the same mass as the reacting components). Shortly after, (1797) J. L. Proust showed that when chemicals join to form compounds the proportions of the reaction (by weight) are always the same. Thus, for example, when hydrogen and oxygen combine to form water the ratio of the two is always 1 part hydrogen to 8 parts oxygen (by weight). Further work by John Dalton established that the fundamental constituent of matter was the atom, and that while each element had atoms different from that of the other elements, all atoms of a given element ware identical. Dalton's results immediately accounted for the findings of Lavoisier and Proust. Dalton's scheme of assigning symbols to the elements provided a method of ordering the myriad of chemical reactions into a systematic structure. His symbols, some of which are reproduced to the right, were eventually replaced by the alphabetical abbreviations we use today.

As more became known of how chemicals combine the underlying structure began to clarify. Avogadro had postulated that equal volumes of gasses at the same temperature and pressure contain the same number of molecules. Later he argued that these molecules might consist of more than one atom. The crowning achievement was by The Russian chemist Dmitry Ivanovich Mendeleyev who in 1869 arranged the known elements (there were 63 at the time) in his periodic table of the elements. The table arranged the elements into six groups according to their valence, which determines how the element combines in a compound.

 It seems amazing that all the work leading up to the compilation of the periodic table of the elements was done without any direct evidence about the existence of either atoms or molecules. No one had seen a molecule, let alone an atom (and an atom is yet to be seen). The scheme of Mendeleyev showed elements align in his table according to valence and, when gaps in atomic weight appeared in his table he predicted the existence of an element to fill the gap. Mendeleyev's table provided a powerful tool for cataloging the known elements and discovering new ones, but the increasing sequence of atomic weights that occurs as one moves through the table was as yet unexplained.

Go to the page on atomic spectra