ATOM (Greek το ’ατομον [to atomon]—an indivisible thing or substance, ’ατομος [atomos]—the idea or form of a thing)—in the philosophy of nature: an object the is the limit for the divisibility of matter; in chemistry: the minimal portion of a chemical element that still manifests the chemical properties of the element; in physics: a balanced system of nucleons and electrons.

THE ATOM IN THE PHILOSOPHY OF NATURE. The idea of the atom appeared for the first time in the philosophy of nature with Leucippus and Democritus. The atom-forms or atom-ideas of Leucippus and Democritus cannot be detected by the senses. They can only be grasped by the intellect according to a similarity to what is corporeally visible. Atoms are qualitatively undifferentiated. They differ only in their quantitative (or geometrical) properties, i.e., shape, order, and position. Atoms are a breakdown of the being-unity of the Eleatics into an infinite number of beings and unities that retain at the same time the most features of being-unity (immutability, eternity, and indivisibility). Emptiness or non-being is found at the same ontological level of reality besides the atom or fullness. Emptiness or non-being is a condition for the motion of atoms. The motion of atoms is subject to strictly deterministic laws that are at the same time the source of the self-identity of atoms. Atoms join together in greater aggregates. To this end they use various geometrical shapes (hooks, openings, and appendages). The larger atoms have been gathered together in the center of the world, while the smaller ones are on the periphery. Epicurus made some essential modifications to Leucippus’ and Democritus’ conception of the atom. He thought that there was a limited number of shapes among atoms, but that atoms of any given shape were infinite in number, whereas in Democritus’ theory atoms also have weight whereby they can fall in the vacuum. Epicurus said that although different kinds of atoms have different weights, they all fall at the same velocity. He also introduced a new conception of parenclisis whereby he was able to move away from the determinism of Democritus and could ascribe a kind of freedom to atoms.

In the Middle Ages the conception of the atom underwent certain modifications. According to William of Conches, everything in the universe remains genetically linked with its archetypes, and through the archetypes with God. The genetic beginning of nature consists in particles or atoms (elementa) that are infinitesimal, incomposite, absolutely simple, beyond the grasp of the senses, and accessible only to the reason (“haec elementa nunquam videntur, sed ratione divisionis intelliguntur”), and they are concentrated in their elements. William males a distinction between atoms (elements) understood in this way and the four elements apparent in sense experience (elementata). The four sensible elements are mixtures of the “elements” that are only intelligible. As a result of opposed processes of thickening (spissatio) and thinning (attenuatio) a transmutation occurs, i.e., the passage of some “elementata” into others, which William uses to explain the variety and changes found in the phenomenal world.

There were important modifications in how atoms were understood in modern philosophy. P. Gassendi rejected the idea that atoms are eternal. Gassendi held that atoms are created and put in motion by God. He also rejected the Epicurean-Lucretian conception of parenclisis (clinamen) and returned to the determinism of Democritus. He also defined the atom as a particle that cannot be broken apart, but whose existence follows from a rational analysis of motion. He also held that groups of atoms that have been joined together permanently and form the germs of things whereby specific (qualitative) features are transmitted. According to Gassendi, the atoms of particular substances have various masses. In order to emphasize that an atom is a particle of a very small mass, this philosopher coined a new expression: molecule. This term was still used interchangeably with the term atom in the early decades of the nineteenth century. Newton thought that the atoms God created were solid, hard, and impenetrable particles that possessed mass.

THE ATOM IN CHEMISTRY. In the seventeenth and eighteenth century’s vision of chemistry, the concept of the atom evolved along with the concept ot the chemical element. The term “atom” was used interchangeably with the term “molecule” and the term “element”, since at that time these concepts were only qualitative. The systematic application of methods of weighing led gradually in the nineteenth century to a more concrete conception and a quantitative description of the molecule and atom. In the late eighteenth and early nineteenth century, scientists recognized Boyle’ hypothesis, that elements are not the vehicles of qualitative or chemical properties, but they are the simplest components of bodies, i.e., they are the ultimate term in chemical analysis. Scientists learned that the properties of a chemical bond are not the sum of the properties of its components. Until the mid- eighteenth century scientists were not aware of the difference between a uniform mixture and a chemical compound. Riding on the wave of the criticism of Aristotle, scientists rejected the four basic qualities (hot, cold, dry, west), combinations of which were supposed to produce the variety of chemical substances. Scientists adopted other criteria for differentiating between chemical elements, i.e., valence, atomic mass, and the configuration of electrons in their external orbits. A. Avogadro’s law had an important influence on the development of the conception of the atom. His hypothesis built upon Dalton’s views and stated that gases under the same conditions of pressure and temperature have an equal number of molecules. Avogadro strictly limited the concept of the molecule and of the atom. The conception of the molecule based on atomic weight fell as the result of the discoveries in the late nineteenth and early twentieth century. With new methods in physics scientists could break down atoms and create chemical particles. The conception of the atom was not longer the ultimate term of analysis. When defining the concept of the atom in chemistry, we should consider the above circumstances. The chemical particle therefore does not mean the same thing as the ultimate physical particle, but it is only the final product of the breakdown of chemical substances using the methods of chemistry.

THE ATOM IN PHYSICS. The discovery of the structure of the atom became possibly due to many works (J. J. THomson, H. Becquerel, P. Curie, M. Curie-Skłodowska, M. Planck, E. Rutherford) in physics in the late nineteenth and early twentieth century. Present views on the construction of the atom come from the theory of the hydrogen atom formulated by N. Bohr in 1913. The theory assumed that the hydrogen atom has one proton around which there is one electron in a circular orbit and it was advanced to provide a solution to contradictions with the laws of classical electrodynamics. Bohr’s model was developed by A. Sommerfeld (1916). Sommerfeld thought that the orbits of electrons could be elliptical, which would explain a certain observed effect called the subtle structure of spectral lines. The Bohrs-Sommerfeld model, although it did not fully explain reality, was the best way to present the microscopic level of reality. A fully adequate description of the construction and properties of the atom first arose in the developing field of quantum mechanics (E. Schrödinger, W. Heisenberg, P. A. M. Dirac). According to this theory, an atom is composed of a nucleus (maintained in unity by nuclear forces) which is almost a point compared to the dimensions of the electron shell. The electron shell, however, does not have clearly defined boundaries, since the numeric values that characterize the dimensions of the atom depend on how they are assigned. The atom’s nucleus is composed of protons and neutrons. The (positive) value of the charge of the nucleus (the atomic number, i.e., the number of protons) determines the element to which a particle belongs (it assigns an ordinal number in the periodic system of elements), and this in turn defines its chemical properties. In fact, however, chemical properties and the most physical properties are determined by the structure of the atom’s electron shells. According to quantum mechanics, the state of each electron in electron shells is described by so- called quantum numbers. Forces of electrostatic attraction operate between the atomic nucleus and the electrons in particular electronic shell and these forces maintain the equilibrium of the system of the nucleus and electrons. The mass of an atom is assigned in practical terms by the mass of its nucleus, since the mass of an electron is around 1840 times smaller than the mass of a proton or neutron. Atoms of the same element, however, may differ in mass (the same number of protons, but a different number of neutrons) and such atoms are called isotopes.

K. Lasswitz, Geschichte der Atomistik vom Mittelalter bis Newton, I–II, H-L 1890; L. K. Nash, The Atomic-Molecular THeory, C 1950; F. Solmsen, Epicurus and Cosmological Heresies, American Journal of Philology 62 (1951), 1–34; A. G. M. van Melsen, From Atomos to Atom, the History of the Concept Atom, Pi 1952; V. E. Alfieri, Atomos Idea, Fi 1953; G. Cogniot, Pierre Gassendi, restaurateur de l’épicureisme, La penseé 63 (1955); Robert Boyle on Natural Philosophy, Bloomington 1965; F. Greenaway,John Dalton and the Atom, Lo 1966; D. J. Furley, Two Studies in the Greek Atomists, Pri 1967; E. Patterson, John Dalton and the Atomic Theory, GV 1970; A. Thackray, Atomism and Powers. An Essay on Newtonian Matter-Theory and the Development of Chemistry, C (Mass.) 1970; Infinity and Continuity in Ancient and Medieval Thought, It 1982; R. Sorabji, Time, Creation and the Continuum, It 1983; R. Mierzecki, Historyczny rozwöj pojęć chemicznych [Historical development of chemical concepts], Wwa 1984, 90–138; S. Makin, The Indivisibility of the Atom, AGPh 71 (1989), 125–149; Atom, Pneuma, and Tranquility. Epicurean and Stoic Themes in European Thought, C 1991; A. Estany, Louis Proust y la revolución ontológica de John Dalton, Arbor 152 (1995), 111–135; R. Kayser, Die Urbewegung der Atomos bei Keukipp und Demokrit, Antike Naturwissenschaft und ihrer Rezeption 6 (1996), 17–40; T. O’Keefe, The Ontological Status of Sensible Quantities for Democritus and Epicurus, AncP 17 (1997), 119–134.

Zenon E. Roskal

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