John Dalton's Atomic Theory

Through his study of gases, John Dalton arrived at some of the most important theoretical concepts in modern chemistry. He established a system whereby relative particle weights can be obtained from available chemical data. In addition, he devised a simple system of rules for chemical combination. Dalton also provided a framework for a system of atomic symbolism. His atomic theory is very useful for explaining a wide variety of physical and chemical phenomena.

Some have called John Dalton the father of the atomic theory of chemistry (1:83). His exact birth date is unknown. By his death in 1844 at the age 78, Dalton had devised some of the most fundamental ideas of modern chemistry (3:984). Indeed, many view the scientist as one of the founders of modern chemistry.

John Dalton was not the first investigator to hypothesize on the atomic nature of matter. Early nineteenth century atomistic ideas can be traced back several centuries. However, while atomism may be "as old as the Greeks," it wasn't until 1417 that the concepts were reintroduced to European thought. That year marked the rediscovery of Lucretius' philosophical exposition, De rerum natura. By the late sixteenth century, this poem on atomism had been widely disseminated. By the eighteenth century, atomic ideas were commonplace. While such concepts were often vague and rarely quantified, they did attract the attention of chemists (4:106-121).

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ht. Similarly, the investigator began to consider the number of atoms in a given volume. This research ultimately extended beyond gases, to include both liquids and solids (4:138-140). In order to explain the elastic properties of matter, Dalton also hypothesized that atomic particles were surrounded by an "atmosphere of heat," or a "caloric (4:106-121)." He claimed that the caloric was similar to the atmosphere of air surrounding the earth and the other planets. Dalton claimed that this caloric was mutually repulsive. Therefore, when heat is added to a gas at constant pressure, the central gas particles remain unchanged but there is an increase in caloric. Such a mechanism causes the heated gas to occupy a larger volume. Likewise, when gas is cooled, there is a diminution of caloric with concomitant volume contraction (4:106-121). According to Dalton's theory, self-repellent calorics only repel atoms of their own kind. This provides a mechanism by which one gas might diffuse through another. Within such mixtures, "every atom of each gas acts as the center of repulsion to the proximate particles of its own kind, and disregards the atoms of other gases (4:106-121)." Initially, Dalton's central particle was similar to

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