Atomic and molecular theory

 Atomic and molecular theory

Lavoisier’s set of chemical elements, and the new way of understanding chemical composition, proved to be invaluable for analytic and inorganic chemistry, but in a real sense the chemical revolution had only just begun. Around the turn of the century, the English Quaker schoolteacher John Dalton began to wonder about the invisibly small ultimate particles of which each of these elemental substances might be composed. He thought that if the atoms of each of the elements were distinct, they must be characterized by a distinct weight that is unique to each element. Although these atoms were far too small to weigh individually, he realized that he could deduce their weights relative to each other—the ratio of the weight of an atom of oxygen to one of hydrogen, for instance—by examining reacting weights of macroscopic quantities of these elements. In fact, the laws of stoichiometry (combining weights of elements) were just then being developed, and Dalton used these regularities to justify his inferences. His first discussion of these issues dates to 1803, and he presented his atomic theory in the multivolume New System of Chemical Philosophy (1808–27).

Dalton’s atomic theory was a landmark event in the history of chemistry, but it had a crucial flaw. His procedure required that one know the formulas of the simple compounds resulting from the combination of the elements. For example, analytical data of that day indicated that water resulted from the combination of seven parts by weight of oxygen with one part of hydrogen. If the resulting water molecule was HO (one atom of each element combining to form a molecule of water), then the weight ratio of the atoms of these elements must be the same, seven to one. However, if the formula were H2O, then the weight of an oxygen atom would have to be 14 times the weight of a hydrogen atom. There was simply no way to determine molecular formulas at that time, so Dalton made assumptions based on the simplicity of nature. He chose HO as his water formula and, therefore, seven as the relative atomic weight of oxygen.

In the following years, several leading chemists adopted essential elements of Dalton’s theory, but many objected to the hypothetical elements just described; some also doubted the very possibility of investigating the world of the invisibly small. In 1808 the French chemist Joseph-Louis Gay-Lussac discovered that when gases combine chemically, they do so in small integral multiples by volume. Three years later the Italian physicist Amedeo Avogadro argued that this fact suggested that equal volumes of gases contain equal numbers of constituent particles (Avogadro’s law), physical conditions being the same. This idea provided a physical method of determining certain molecular formulas. For instance, Gay-Lussac had pointed out that exactly two volumes of hydrogen combine with precisely one of oxygen to form water. If Avogadro was right, the formula for water had to be H2O. But this line of reasoning also led to the uncomfortable notion that elementary gases had polyatomic molecules (O2, H2, and so on), and therefore many chemists rejected Avogadro’s hypotheses.

By far the greatest of the early atomists was the Swede Jöns Jacob Berzelius, who accepted parts of Avogadro’s ideas and developed an elaborate version of chemical atomism by 1826. It was Berzelius who in 1813 had proposed the alphabetic system for denoting elements, atoms, and molecular formulas, and the use of formulas as an aid for studying chemical composition and reactions began to blossom about 1830. However, different chemists were still making different assumptions regarding the formulas of simple compounds such as water, and so, for decades, various inconsistent systems of atomic weights and formulas were in use in the various European countries.

Berzelius also developed a theory of chemical combination based on the electrochemical studies that the invention of the battery (1800) had spawned. He became convinced that all molecules were held together by the Coulomb force, the electrostatic attraction between oppositely charged objects. (Berzelius assumed that a molecule’s constituent atoms or groups of atoms were not neutral, and he called these charged components radicals.) This theory of electrochemical dualism worked well with inorganic compounds, but organic substances seemed anomalous. Particularly in the 1830s, when chemists learned how to replace the hydrogen of organic compounds with chlorine atoms, Berzelius’s theory appeared to be threatened—after all, hydrogen and chlorine had opposite electrochemical characteristics, yet the substitution seemed to make little difference in the properties of the compounds. In the 1840s and ’50s, extensive debates over rival systems of chemical atomism and over electrochemical dualism enlivened the journal literature.