Atomic theory," wrote Bernard Pullman, "is the most important and enduring legacy bequeathed by antiquity." Indeed, this the atomic theory provides a brilliant example of pure speculation which develops into a universally accepted scientific theory. First developed by the Greek philosophers Leucippus and his follower Democritus (c.430-c.370 b.c.), atomic theory remained purely speculative until the time, in the early twentieth century, when scientists started probing atomic structure through experimental methods. Furthermore, Greek atomism is a striking departure from other cosmogonic theories, such as the idea, suggested by Thales (c. 624-545 b.c.), that everything is derived from water, since atomism did not in any way rely on empirical observation.
Leucippus, who probably lived in the fifth century b.c. is widely considered as the father of atomic theory. Because no writings by Leucippus have come down to us (in fact Epicurus, a prominent atomist, denied his existence), his ideas are difficult to separate from, those of Democritus. According to Leucippus and Democritus, the material world consists of invisible and indivisible (in Greek: atomos) corpuscles. Atoms are hard, compact, incompressible, infinite in number, and in permanent motion. However, atoms are not all identical: the differ according to shape, arrangement, and position. Building a comprehensive theory of reality on the basis of atoms, Democritus used atoms to explain a wide variety of phenomena, including the structure of the inanimate world, life processes, as well as human memory and perception. No less important than the obvious idea of the atom is the concept of void, which Democritus introduced to explain atomic movement. Later Greek philosophers, such as Aristotle (384-322 b.c.), rejected atomism because they found the idea of a void unacceptable. Atomic theory was further developed by Epicurus (c.341-270 b.c.), who theorized that atoms had weight and moved at a steady speed. His best-known contribution to atomism, however, is the idea of the clinamen, or swerve. According to Epicurus, atoms had a tendency to suddenly, and randomly, deviate from a seemingly predictable downward trajectory. These deviations, Epicurus taught, account for the variety of material phenomena. The atomism of Epicurus inspired the Roman poet Lucretius Carus (c. 99-55 b.c.) to compose his De rerum natura, a poetic exposition of atomism which is regarded as one of the great works of Roman literature. Epicurus has been criticized, somewhat anachronistically for his atheism, mainly because of his materialism, according to which even the human soul, which consists of atoms, dies with the body. Epicurus never questioned the existence of a divine reality, manifested by immortal gods; however, he taught that the gods were indifferent to the human realm.
Christian thinkers condemned the atomic theory, because they equated materialism with atheism. This bias was strengthened in the Middle Ages, when Christian theology incorporated the teachings of Aristotle, who not only rejected the concept of void, an integral element of the atomic theory, but also accepted, in modified, the theory of four elements. However, atomic theory, although neglected, was not completely forgotten in the Middle Ages. For example, Adelard of Bath, who lived in the twelfth century, defined atomism as a rational explanation of the material world. In a more direct fashion, William of Ockham (1300-1350) stated that the physical reality consisted of elementary particles.
The most prominent representative of atomism in Renaissance was Giordano Bruno (1548-1600). Introducing elements of Platonic idealism into atomic theory, he expanded the traditional definition of the atom, adding specific spiritual attributes, such as the possession of a soul, in an effort to reconcile a corpuscular theory of matter with his grand vision of an essentially spiritual universe. It is ironic that this passionate believer in the divine origin of scientific knowledge was burned as a heretic in Rome.
In the seventeenth century, as scientists and philosophers started turning to empirical research in order test their theories, atomists approached atomic theory as a key to nature's secret. Thus Robert Boyle (1627-1691), who was a chemist, strove to establish the connection between particular types of atoms, which he imagined appeared in specific shapes, and certain perceived characteristics of material substances. Isaac Newton (1642-1727) accepted atomism, but raised the question of creation as a result of pure chance which traditional atomic theory implied. For Newton, creation could only the work of a Divine Intelligence, and he accordingly adapted atomism to fit his conception of God's role in creation; however, Newton's universal law of gravitation, which applies to both macroscopic and microscopic phenomena, opened new theoretical vistas which greatly enhanced the development of atomic theory. For example, Rudjer Boskovic (1711-1787), building on Newton's theory of universal gravitation, created an atomic theory that was clearly ahead of its time. Namely, Boskovic, who accepted Newton's gravitation theory as valid in macrocosmic realm, posited that, as the distance between physical objects diminishes, attraction is replaced by a repulsive force. Thus, while attraction provides the atomic cohesion needed for the construction of physical objects, a repulsive force keeps individual atoms at a certain distance from each other. It is Boskovic's conception of an atom--lacking spatial extension, resembling a geometrical point--that conjures up the world of subatomic particles, discovered in the twentieth century. Yet Boskovuc's world view, like Newton's, did not allow the possibility of a universe without God.
In 1789, in his Elementary Treatise of Chemistry, Antoine-Laurent Lavoisier (1743-1794), widely considered the father of modern chemistry, defined an element as a substance that cannot be chemically analyzed. Lavoisier's fundamentally analytical approach to chemistry set the stage for the great breakthroughs in atomic theory in the nineteenth century. Thus, John Dalton (1766-1844), in A New System of Chemical Philosophy (1808), asserted, building on Lavoisier's observation that each element was a unique substance, that atoms forming a particular element were similar and had the same weight. In 1815, having observed that the densities of several gases were whole number multiples of the density of hydrogen, William Prout (1785-1850) theorized that the atomic weights of different elements were whole number multiples of the atomic weight of hydrogen. Throughout the nineteenth century, scientists studied the behavior of gases to strengthen the case for atomic theory. For example, the kinetic molecular theory, proposed independently by James Clerk Maxwell (1831-1879), in 1859, and by Ludwig Boltzmann (1844-1906), in the 1870s, followed the hypothesis that gases consisted of atoms and molecules in constant motion. Furthermore, Dmitri Mendeleev (1834-1907) developed his periodic table of elements by demonstrating the connection between, on the one hand, an element's atomic weight, and its chemical properties and physical characteristics, on the other.
The doctrine of an atom's indivisibility was shattered in 1897, when J. J. Thomson (1856-1940), published his discovery that cathode rays, which are emitted by the cathode, or negative electrode, in a gas-filled tube through which electricity is discharged negatively charged particles whose mass was a mere fraction of an atom's. When Thomson discovered that these particles, named electrons by George Johnstone Stoney (1826-1911), were identical regardless of the gas from which they emanated, he realized that what he had found were elements of an atom's inner structure. The existence of electrons was confirmed by numerous subsequent findings, including Marie-Sklowska-Curie's (1867-1934) demonstration that the beta-particles, emitted by decaying uranium atoms, which Henri Becquerel (1852-1908) had identified in 1896 as radioactivity, were in fact electrons. Thus, at the end of the nineteenth century, two axiomatic features of the atom were invalidated: indivisibility and stability.
In 1902, Lord Kelvin (1824-1907), in an attempt to describe an atom's structure, proposed a model according to which electrons were evenly distributed throughout a positively charged space, effectively creating a balanced state. Two years later, J. J. Thomson modified Kelvin's model, having the electrons move in concentric circle within a positively charged spheric space. However, the Kelvin-Thomson model was challenged by Ernest Rutherford (1871-1937), who noted, on the basis of experiments in which thin metal foil was bombarded by alpha particles, or positively charged ions, that the characteristic scattering pattern of the alpha particles indicated the existence of a positively charged nucleus within the atom. In other words, the alpha particles usually encountered no resistance within the atom, as if the atom were empty, only occasionally ricocheting back. If the Kelvin-Thomson model had been accurate, the alpha particles would have been repelled by a positively charged space. In 1911, Rutherford explained the results of these experiments: most of the atom is, in fact, empty space, the nucleus representing a minuscule fraction of an atom's volume. Rutherford's model provided the foundation for several important discoveries, including the positively charged particle, or particles, constituting the nucleus, which Rutherford named proton in 1920. In 1932, James Chadwick demonstrated the existence of neutral particles, or neutrons in the nucleus. Also in 1932, Carl Anderson (1905-1990) discovered the positron, or positively charged electron, the first known particle of antimatter, anticipating future discoveries of other antiparticles. Building on the pioneering work of Harry Moseley (1887-1915), who was killed at Gallipoli, scientists also defined the fundamental concept of atomic number as the number of protons in a given atom's nucleus. In addition, concepts such as atomic number and atomic mass (number of protons plus number of neutrons) enabled scientists to explain the phenomenon of isotopes, or different manifestations of a particular chemical element. For example, with the atomic number remaining constant, different isotopes were defined as consisting of atoms with a varying atomic mass.
However, there were several problems with the Rutherford model, as Rutherford himself admitted. For example, while the planetary model worked for the macroscopic world, in which a planet's orbit is maintained by the equilibrium between gravitation and the centrifugal force, within the atom, where the principal force is the attraction between positively and negatively charged particle, an electron orbiting around the nucleus is bound to gradually lose energy, eventually spiraling into the nucleus. Furthermore, the planetary model fails to explain the particular spectral lines, or characteristic lines of the light spectrum, which particular atoms emit when exposed to heat. If the electron kept losing energy, the emitted radiation would not be constant, effectively excluding the possibility of consistent light patterns.
The atomic model which replaced Rutherford's was based on Max Planck's (1858-1947) theory of quanta. In 1900, while observing light emitted by a heated object at various temperature, Planck observed a certain regularity in the vibration of a heated object's atom. In fact, the frequency of a vibrating atom is always the value of a physical constant, later named Planck's constant, multiplied by a whole number. In other words, energy can only be emitted in discrete chunks.
In 1913, Niels Bohr (1885-1962) used Planck's quantum theory to do develop a new theoretical model of the atom. Bohr's model was based on two fundamental postulates: an electron has specific energy values, or levels; and an electron can change its energy value only by jumping from one energy level to another. According to Bohr, transition between energy levels explained the line spectrum: when an electron dropped to a lower energy level, the lost energy was emitted in the form of a photon, or particle of light. Furthermore, Bohr explained the atom's stability by positing that an electron can rise to a higher energy level: in other words, a gradual loss of energy, eventually leading to an electron's collapse into the nucleus, simply does not occur. An electron's ability to ascend to a higher energy level also explain the phenomenon of light absorption: by absorbing a photon an electron jumps to a higher energy level.
Initially, each electron in an atom was assigned three quantum numbers: the principal quantum number (describing an electron's energy), the angular momentum quantum number (describing the shape of an electron's path), and the magnetic quantum number (describing an electron path's orientation in space). Wolfgang Pauli (1900-1958) introduced a fourth number, the spin quantum number (describing an electron's movement similar to a spin on its axis). Magnetism caused by electron spin was fist observed by Otto Stern (1888-1969) and Walther Gerlach (1889-1979) in 1921. In 1924, Pauli formulated his famous exclusion principle, according to which no two electrons in an atom may have all four identical quantum numbers.
The application of quantum theory to the behavior of subatomic particles opened a filed known as quantum mechanics, also termed wave mechanics, because scientists expanded quantum theory to include wave phenomena. Thus, in1923, Louis de Broglie (1982-1987), generalized Albert Einstein's (1879-1955) insight that light has both particle and wave properties by positing that particles of matter have wave properties. Four years later, Lester Germer (1896-1971), Clinton Davisson (1881-1958), and J. J. Thomson's son George Paget Thomson (1892-1975) demonstrated the wave property of electrons when they diffracted an electron bean by using a crystal. This discovery enabled Ernest Ruska (1906-1988) to develop the first electron microscope in 1933. De Broglie formulated a mathematical equation to define the wave behavior of particles; however, since his equation only applied to particles in a force-free environment, it could not work for electrons, which are affected by the attractive force of the nucleus. However, in 1926, Erwin Schrödinger (1887-1961) developed a theory which attempted to explain the wave property of electrons. According to Schrödinger, the movement of an electron in relation to the nucleus could not be described as an orbit: in order to conceptualize any such movement, one would need to know both the position and momentum of a particle. This, as Werner Heisenberg (1901-1976) demonstrated in 1927, is impossible (Heisenberg's uncertainty principle). True, Schrödinger attempted to mathematically track an electron's movement by using a wave function; however, this mathematical function, in light of the uncertainty principle, only yielded the probability of encountering an electron at a given spot. Instead of the misleading term orbit, scientists started using the word orbital to describe an electron's movement. Scientists have used mathematical to describe orbitals, but in reality their exact shapes cannot be known. "Nevertheless," as Lothar Schäfer has written "we say that these wave forms exist, because the results of their interference are apparent in observable phenomena. For example, an important type of interference, and the basis of all chemistry, is that between different atoms when they are forming a chemical bond. The results are apparent in the observable properties of molecules; their structures, for example, depend directly on the atomic wave forms by whose interference they are created."
While the dogma of an atom's indivisibility was refuted in at the end of the nineteenth century, the dogma of invisibility endured almost a century longer, fueled by claims that the creation of an appropriate microscope was precluded by the laws of physics and human physiology. However, in 1981, Gerd Binnig (1947-) and Heinrich Rohrer (1933-) invented the scanning tunneling microscope, which enabled scientists to see individual atoms. In 1986, Binnig and Rohrer received a half of the Nobel Prize for physics; the other half went to Ruska, who invented the first electron microscope.
As scientists continued studying the subatomic world, they realized that, while electrons remained definable as elementary particles, neutrons and protons may be defined as constructs of smaller particles. For example, according to Murray Gell-Mann (1929-) particles such as neutrons or protons consist of smaller particles, which he named quarks. Since, for example, a proton has a charge of +1, a quark would have a fractional charge.
One of the great accomplishments of contemporary atomic theory, as Pullman, has written, is the "remarkable effort to synthesize the two major theoretical contributions of the twentieth century, namely, the theory of relativity, with its fundamental law of equivalence between mass and energy, and quantum mechanics." According to Pullman, the paradigm emerging as a result of this synthesis is the relativistic quantum theory of fields, according to which reality can be defined in terms of interacting fields. "Fields," Pullman asserted, "are the ultimate reality, and there are as many fundamental fields as there are elementary particles."
Among the significant corollaries of the theory of fields, which offers a unified theory of reality, is a profound revision of the concept of vacuum. Thus, if reality is defined as a system of interconnected fields, the dichotomy, postulated by Democritus, between atoms and a vacuum in which they exist becomes meaningless. According to Pullman, vacuum "is a latent state of reality, while matter, made of elementary particles, is its actualized state." Finally, while many scientists accept the field paradigm as intellectually satisfying, the consensus among researchers is that further work in atomic theory will lead to many surprising discoveries.
