John Dalton Proposes His Atomic Theory and Lays the Foundation of Modern Chemistry

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John Dalton Proposes His Atomic Theory and Lays the Foundation of Modern Chemistry

Overview

As the nineteenth century dawned a significant problem that remained in the chemical sciences was the ultimate nature of matter. Was matter continuous and therefore had no finer structure or was it discontinuous and thus made of tiny particles? The chemical revolution due to the work of Antoine Lavoisier (1743-1794) and his circle that had occurred in the last two decades of the eighteenth century had clarified the concept of what elements are, developed a comprehensive and consistent vocabulary of chemistry, and led to the introduction of quantitative methods in chemical investigations. However, to fully understand the nature of chemical reactions one needed to have a way to visualize how the elements combined together. The atomic theory of matter as proposed by John Dalton in his New System of Chemical Philosophy (Part I,1808; Part II,1810) was the first successful attempt to solve this problem.

Background

The concept that matter may ultimately be composed of particles originated in Greek natural philosophy. In the fifth century b.c. Democritus (c. 460-370 b.c.) proposed that matter was composed of individual indestructible particles (called "atoms" in Greek for "uncuttable") and that the size and shape of these particles were responsible for the properties of matter. The atomic theory of the Greek philosophers lacked any evidence based upon observation, measurements, and testing by experiment. These ideas, though interesting, could not be considered a scientific theory.

The atomic concept was rejected by most Greek philosophers, particularly Aristotle (384-322 b.c.), because of the paradox that these atoms had no sensible properties—yet they had to be responsible for all the properties of matter that one could sense, such as an object being hot. The concept of atoms would also mean that there were possibly an infinite number of primary substances in nature. This was in direct conflict with the idea of the four elements—earth, air, fire, and water—being the primary building blocks of everything on Earth. A further problem was that if matter was particulate, then there would be spaces or voids between matter, which would make motion impossible. Finally, if matter was made up of atoms, then a purely mechanical explanation of human actions and behavior would be possible. By the Middle Ages such an explanation was rejected because it introduced the possibility that human actions were not set in motion by a divine being. A revival of atomism would have to wait until the rise of experimental science in the seventeenth century.

Robert Boyle (1627-1691) in his Skeptical Chemist (1661) proposed that all matter is composed of solid particles that can be rearranged to form new substances. What differentiated these different types matter was their size, shape, and structural pattern. Isaac Newton (1642-1727) in his Opticks (1704) also proposed a particulate view of matter, and he further proposed that there were strong short range forces that existed between these particles that could be of an attractive or repulsive nature. This was used by him to explain why some chemical reactions occurred and others did not. As was the case with the Greeks, Boyle, Newton, and others had no evidence to back up their claims. However, their view that matter is particulate signaled an increasing consensus among scientists of the era—a consensus that would make the theory proposed by Dalton much more readily acceptable.

John Dalton (1766-1844) was a most unlikely person to develop the atomic theory. Born into a devout Quaker family in a rural area of northwest England, he was drawn early in life to an interest in the natural sciences. His formal education was spotty and he was basically self-taught. He was for a time the equivalent of a high school teacher in Manchester, England. He quit classroom teaching in 1800 to provide private instruction in the sciences and mathematics in Manchester, which he did for the balance of his life.

The impetus for the development of the atomic theory was Dalton's life-long interest in meteorology and the study of gases. This interest had developed from his association in his youth with a fellow Quaker, John Gough, who provided Dalton with most of his formal education in the sciences and mathematics.

Dalton developed his atomic theory as a way of trying to answer certain questions about the atmosphere. In the eighteenth century it was shown that the atmosphere was a mixture of gases rather then a single substance. The identities of many of these gases had only had been recently established. Dalton wondered if the atmosphere was a simple mixture of gases such as oxygen, nitrogen, carbon dioxide, and water vapor, or perhaps if there was some type of chemical reaction that occurred between these gases. Since the atmosphere appeared to be a homogenous mixture of gases, the consensus view at the end of the eighteenth century was that the various components were chemically combined and dissolved in the water vapor. Further evidence for this view was that if the atmosphere was a simple (i.e. physical) mixture of components, then one would expect that the various gases would settle out according to their weights, with the heaviest closest to the surface and the lightest on top. Since this is not what was found, it seemed logical that air was a chemical compound.

Dalton believed that the atmosphere was a physical mixture based upon his belief that water vapor could not be combined chemically with the gases in the air. Dalton viewed matter as composed of spherical particles and believed that these particles or atoms contained a shield of heat around them. This was essential for Dalton to explain why unlike particles tended to repel each other and thus produce a physical mixture of gases in the atmosphere. The idea of the shell of heat, or "caloric" as it was called, was incorporated from Lavoisier's model of the gaseous state and the belief that heat was a material element. Dalton used the term atoms for these particles in order to show that the original concept had originated in Greek natural philosophy.

Each atom in nature had its own size, which was a function of the volume and the radius of its shell of heat. Dalton formulated these ideas between 1801 and 1803, but evidence was lacking. This was supplied by a close friend of Dalton in Manchester, William Henry (1774-1836). Henry found that if you kept the temperature of a liquid such as water constant, the amount of a non-reactive gas that could be dissolved in it would increase as you increased the pressure place upon it. This led Dalton in 1803 to suspect that it was the weight of the particles of the gases that was the key determining factor. He measured the relative weights of various gases from their composition and presented his first table of relative atomic weights for a variety of gases and other substances.

Up to this point Dalton had shown little interest in chemistry, but by 1804 he realized that if atoms were considered to be the ultimate particle in nature and that each atom had its own particular weight, this could explain observations that had been made concerning the composition of compounds. As methods for the analysis of compounds had been refined, it was found that a compound always had the same composition—no matter if it was obtained from natural sources or made synthetically. Thus, if one analyzed rain water against water made by combining hydrogen and oxygen together in a laboratory, one would find that there would be 11.2% hydrogen and 88.8% oxygen in each case. This became known as the law of definite proportions, and it was the atomic theory that showed why this was so. If hydrogen and oxygen combine in a 1:1 ratio, then an atom of oxygen must be eight times heavier than an atom of hydrogen in order to get the constant composition of water.

Impact

The ability of Dalton's atomic theory to explain the law of definite proportions was only the beginning of its impact on the field of chemistry. Another chemical problem that Dalton was able to solve using the atomic theory was the observation that a particular element such as nitrogen, for example, could combine with oxygen and form a series of unique compounds containing nitrogen and oxygen. Analysis of these compounds showed that there was a regular relationship between the amount of nitrogen that combined with oxygen. Thus, if you have a fixed amount of nitrogen, the amount of oxygen combined would stand in a series of whole numbers—i.e. 1:1,1:2,1:3, and so on. This came to be known as the law of multiple proportions and was a puzzle until Dalton. He was able to explain multiple proportions by assuming that an atom of nitrogen could react with one, two, or more atoms of oxygen to form a series of compounds.

Dalton's explanation of multiple proportions was used by William Hyde Wollaston (1766-1828) and Thomas Thomson in 1807 to explain the relationship of potassium bicarbonate and potassium carbonate. Thomson, who had discussed the atomic theory with Dalton in 1804 when he visited Manchester, was so impressed by how it solved his problem that he arranged for Dalton to lecture at the universities of Edinburgh and Glasgow on his views of chemical reactions using the atomic theory. These lectures given in 1807 were well received and ultimately formed the basis for the New System of Chemical Philosophy, the first part published in 1808. The atomic theory itself only occupied five out of the 916 pages of this first part of the New System.

In general, the impact of the atomic theory can be summarized as follows:

1) The definition of an element as being made of atoms, and the idea that each atom has its own unique properties, led at last to a clear understanding of what an element is.

2) Since there were no limits on the number of different atoms possible in nature, then it seemed perfectly reasonable that there were elements that had not yet been discovered. This led to the search for new elements, a search that occupied many chemists during the nineteenth century and that led to the discovery of numerous elements.

3) Since elements are made of atoms and many different elements seem to have similar chemical properties, this raised the question of why certain groups of elements were similar in nature while others differed greatly. This contributed to the development of schemes to classify elements, an effort that culminated in Dmitri Mendeleyev's (1834-1907) first periodic table of the elements in 1869.

4) Since atoms combine together to form molecules, the atomic theory stimulated investigations as to the reasons that certain atoms combine together and others do no. This led to the development of theories of chemical bonding in the nineteenth century.

Dalton's concept that matter was made up of these incompressible particles surrounded by an atmosphere of heat was difficult for nineteenth-century chemists to accept. However, if one used the concept of atomic weight as a tool, then the synthesis of compounds was made much easier. The atomic model in which atoms always retained their identity showed why the law of conservation of mass existed in nature and why transmutation of the elements could not occur: an atom of lead could never be transformed into an atom of gold, since atoms always maintained their identity no matter what you did to them. The atomic theory maintains that there are as many atoms as there are elements in nature and that within an element all the atoms are the same.

There was much skepticism concerning the atomic theory for several reasons. The most obvious was the inability of Dalton to physically demonstrate the presence of atoms in matter.

Even if you could not show their physical presence, however, the relative weights of the atoms were useful as a means of chemical synthesis. It was in this light that the atomic theory had its greatest usefulness in the first half of the nineteenth century. When Dalton was awarded the Royal Society medal in 1826, it was for his "development of the theory of definite proportions, usually called the atomic theory of chemistry."

MARTIN D. SALTZMAN