The Birth of Modern Atomic Theory
Published January, 2021, in Academia Publishing
At the beginning of the nineteenth century, English chemists Humphry Davy and John Dalton raised anew the age-old question the Greeks had asked: What are the ultimate constituents of matter? That question now became the core of the science of chemistry.
In 1808, Dalton revived Democritus’s idea that matter is composed of atoms, proposing a chemical atomic theory based on what he called the Law of Constant and Multiple Proportions, which grew out of his interest in Earth’s atmosphere. The air we breathe, he knew, was composed of different gases. How were they distributed in the air? Was air, for example, a homogeneous mixture? In studying gaseous mixtures, Dalton was impressed by one striking physical characteristic: the heavier the gas, the more it was absorbed by water. This observation suggested an idea to him: The weights of different particles in a mixture of gases were different.
Dalton’s chemical atomic theory offered an explanation of what was going on when chemical combinations occurred. Operationally, the theory also explained why chemical substances, when combined in the same proportions and under the same circumstances, produced the same compounds.
Dalton’s theory had two postulates: first, that substances were composed of individual atoms; and second, that atoms making up a particular substance each weighed the same, and differed in weight from atoms making up a different substance. Not only did Dalton ( 1766-1844) consciously consider the internal structure of matter, but he also provided the criteria for distinguishing between different elements: weight and size. Dalton outlined in his New System of Chemical Philosophy (part I, 1808; part II, 1810) his procedure for assigning an atomic number for all the known elements—21 were known at the time. All of his numbers were relative to the weight of hydrogen, to which he assigned the number 1.
Dalton’s rival for the informal title of England’s greatest chemist was the maverick and charismatic Sir Humphry Davy, a professor of chemistry at The Royal Institution, in London. He embraced theories not in vogue and also speculated about the makeup of ammonia and water, whose compositions were considered well-established experimentally. Yet despite his unorthodoxy, he made stunning discoveries.
He carried out a brilliant chain of experiments leading to the isolation of potassium and sodium in 1807. In the following year, he isolated barium and calcium, the metals of the alkaline earths. He is hailed for demonstrating that chlorine was an element, not a compound, for predicting the existence of the chemically-related element, fluorine; and with a discovery made independently by the French chemist Joseph Louis Gay-Lussac establishing the elemental nature of iodine. The arc of Davy’s chemical achievements includes his work on nitrous oxide, which paved the way for its use in surgery, his electrochemical research, the invention of the arc lamp, the first practical electrical lamp, and a safety lamp for coal miners.
Davy learned chemistry by studying the writings of the French nobleman and chemist Antoine-Laurent de Lavoisier (1743-1774). By the time of his death, Lavoisier had laid waste to the phlogiston theory of matter. In place of the idea that metals were compounds containing one common principle of inflammability, Lavoisier had proposed instead his theory of oxidation to explain the combustion of metals. Metals were simple substances, not compound substances. Davy was dissatisfied with Lavoisier’s theory, or Dalton’s atomic theory, for that matter.
Davy (1778-1829) argued that the elements were not the simplest units of matter. In his view, a few fundamental particles composed all the simple substances commonly called elements. Why did Davy find Lavoisier’s explanation of the composition of metals unsatisfactory? Lavoisier’s definition of a metal was operational: He defined it in such a way as to be useful in the laboratory, as indeed it was, so long as the pre-eminence of oxygen’s role in chemical reactions was stressed. His definition permitted the recognition of 26 different metallic bodies by 1807. But Davy’s approach to the nature of metals was different. He argued, rather, that the analogy of the properties of the metals, their conducting power, the magnitude of the number representing them, their splendour, the similarity of their crystals, would all lead us to the idea of their not being entirely different kinds of matter; but would rather incline one to suppose that they contain some common element or elements. 1
Davy professed to be a proper Baconian—an impartial gatherer of facts, unprejudiced by theories. The reality was otherwise: He felt that chemistry would not become a “true science” until it adopted a theory in which the elements are not the simplest units of matter.
Although it was an internal explanation of the behavior of matter, Dalton’s atomic theory emphasized the individuality of the elements whose relative atomic weights were tabulated. The theory, in Davy’s opinion, sacrificed the idea of a unity of matter. But the theory was very successful in describing the observed behavior of chemical substances. To many chemists, the empirical generalization it described (the Law of Constant and Multiple Proportions) quickly became indistinguishable from the theory from which the atomic theory was so easily deduced. Davy defended the law but not the theory. He even tried to find an electrochemical interpretation for the law of constant and multiple proportions.
While Davy’s speculations about matter were largely responsible for his distrust of Lavoisier’s antiphlogistic theory, his aim was not simply to tear down the existing system of chemistry; he meant to replace it with a theory that could also account for qualities like acidity. Their ideas, in other words, could coexist.
Davy’s objections to Dalton’s atomic theory ran deeper. He maintained until the end of his life the position that his use of “proportionate numbers” were preferable to Dalton’s relative atomic weights. Dalton’s use of the term “element” in his atomic theory precluded Davy’s dream of what he called “a real indestructible principle” of matter ever being realized. Dalton, he wrote, “is too much of an Atomic Philosopher; and in making atoms arrange themselves according to his own hypothesis, he has often indulged in vain speculation…the truly useful part of his doctrine…is perfectly independent of any views respecting the ultimate nature either of matter or its elements.”2
Although not the only chemist of his generation to disagree with Dalton’s model of the structure of matter, Davy’s arguments were the opening salvos in the ensuing debate among 19th century chemists about whether the atom itself had an internal structure.
1Quoted in J. R. Goodstein, “Humphry Davy: Chemical Theory and the Nature of Matter” (Ann Arbor, Mich., University Microfilms, 1969), p. 44.
2 Personal Notebook 46, dialogue 3; The Collected Works of Sir Humphry Davy, Bart., edited by his brother, John Davy (London, 1839-1840), Vol. 5, p. 330, footnote.
See also the recent biography by David Knight, Humphry Davy, Science & Power (Oxford UK, 1992); Lavoisier’s laboratory work on airs is explored in Frederick L. Holmes, Antoine Lavoisier: the Next Crucial Year (Princeton, 1997); Eric Scerri explores the use of the term “element” in The Periodic Table: Its Story and Its Significance (Oxford, 2019).