ATOM

smallest unit into which matter can be divided without the release of electrically charged particles. It also is the smallest unit of matter that has the characteristic properties of a chemical element. As such, the atom is the basic building block of chemistry. Most of the atom is empty space. The rest consists of a positively charged nucleus of protons and neutrons surrounded by a cloud of negatively charged electrons. The nucleus is small and dense compared to the electrons, which are the lightest charged particles in nature. Electrons are attracted to any positive charge by their electric force; in an atom, electric forces bind the electrons to the nucleus. It is easier to describe an atom mathematically than conceptually, and so physicists have developed several models to explain its various characteristics. In some respects, the electrons in an atom behave like particles orbiting the nucleus. In others, the electrons behave like waves frozen in position around the nucleus. Such wave patterns, called orbitals, describe the distribution of individual electrons. The behaviour of an atom is strongly influenced by these orbital properties, and its chemical properties are determined by orbital groupings known as shells. This article opens with a broad overview of the fundamental properties of the atom and its constituent particles and forces. A more mathematical and technical discussion of its structure and nucleus is provided in subsequent sections. Included too is a historical survey of the most influential concepts about the atom that have been formulated through the centuries. For additional information pertaining to nuclear structure and elementary particles, see subatomic particles. smallest unit into which matter can be divided without charged particles being released. It also is the smallest unit of matter with the characteristic properties of an element. During the 5th century BC, Greek philosophers proposed the notion that atoms were hard, indivisible fundamental particles of nature. This view held sway until about 1900, when modern scientific experimentation and mathematical deduction finally gave rise to a more accurate conception of the building blocks of matter. Today, it is known that an atom consists largely of empty space and various constituent particles. Near the centre of the atom is a dense core, or nucleus, comprising protons and neutrons. These so-called nucleons are minute but extremely massive; as a result, the nucleus constitutes more than 99.9 percent of the mass of the entire atom, though it occupies only about 10-14 of the volume. The nucleons cling together to form the nucleus because of an attractive force that they exert on one another when in close proximity. This force is called the nuclear, or strong, force. The nucleus is positively charged, since the protons each carry one unit of positive electric charge while the neutrons carry none at all. As tiny as they are, both the protons and the neutrons are made up of still-smaller particles called quarks. Each nucleon consists of three of these apparently fundamental particles (see quark). The nucleus is surrounded by a diffuse cloud of electrons, particles with a negative electric charge and almost no mass. Because opposite electric charges attract, the negatively charged electrons are bound to the positively charged nucleus. In neutral atoms, the number of electrons equals the number of positive charges on the nucleus (i.e., the number of constituent protons), but any atom may have more or fewer electrons than positive charges and thus be negatively or positively charged as a whole. Such charged atoms are called ions. Within each atom or ion, the electrons are not at rest but seem to move in complicated orbits around the nucleus. The size of an atom and its response to other atoms, particles, and electromagnetic radiation are determined by the arrangement of the orbiting electrons. Atoms combine with each other to form larger structures, such as molecules, by transferring or sharing electrons. The attraction between electrons that are attached to one atomic nucleus and the nuclei of nearby atoms (known as the van der Waals forces) explains much of the mechanism of the formation of liquids and solids. The most significant characteristic of an atom is its atomic number, which is the number of protons in its nucleus. The great importance of this property stems from the observation that all atoms with the same atomic number have nearly identical chemical properties and thus constitute a given element. Not all the atoms of an element have the same number of neutrons in their nuclei. Atoms with the same atomic number but a different number of neutrons are isotopes of that element. Isotopes have identical chemical properties, but they can have very different nuclear properties. These include mass, tendency to become radioactive in nuclear reactions (i.e., radioactivity), and magnetic properties. Additional reading General history Steven Weinberg, The Discovery of Subatomic Particles (1983), concise exposition emphasizing 19th- and early 20th-century discoveries; Emilio Segr, From Falling Bodies to Radio Waves: Classical Physicists and Their Discoveries (1984), and From X-Rays to Quarks: Modern Physicists and Their Discoveries (1980; originally published in Italian, 1976), readable works giving a comprehensive history of thought on the atom from the mid-1850s; Ginestra Amaldi, The Nature of Matter: Physical Theory from Thales to Fermi (1966, reprinted 1982; originally published in 1961), nonmathematical history from the Greeks to 1960; Henry A. Boorse and Lloyd Motz (eds.), The World of the Atom, 2 vol. (1966), reprints of many original papers influential in the development of thought on the atom, highly recommended for its lively and thorough commentary; and Robert P. Crease and Charles C. Mann, The Second Creation: Makers of the Revolution in Twentieth-Century Physics (1986), readable account of physicists and their discoveries in subatomic physics from Bohr to the 1970s. Components and properties of atoms Kenneth S. Krane, Introductory Nuclear Physics (1987), with many applications to other fields of science and technology; Charles Kittel, Introduction to Solid State Physics, 6th ed. (1986), undergraduate-level treatment of the properties of solids emphasizing electron bands; and Robert Eisberg and Robert Resnick, Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles, 2nd ed. (1985), for readers with calculus background but no previous quantum mechanics. For the properties of chemical bonds in terms of electron orbitals, see Linus Pauling, The Nature of the Chemical Bond and the Structure of Molecules and Crystals: An Introduction to Modern Structural Chemistry, 3rd ed. (1960); and C.A. Coulson, The Shape and Structure of Molecules, 2nd ed. rev. by Roy McWeeny (1982). Peter Ring and Peter Schuck, The Nuclear Many-Body Problem (1980), treats the advanced theory of nuclear structure. See also Atomic Data and Nuclear Data Tables, a bimonthly journal devoted to the compilation of the properties of atoms and nuclei. Sharon Bertsch McGrayne George F. Bertsch Development of atomic theory The concept of the atom that Western scientists accepted in broad outline from the 1600s until about 1900 originated with Greek philosophers in the 5th century BC. Their speculation about a hard, indivisible fundamental particle of nature was replaced slowly by a scientific theory supported by experiment and mathematical deduction. It was 2,000 years before modern physicists realized that the atom is indeed divisible and that it is not hard, solid, or immutable. The atomic philosophy of the early Greeks Leucippus of Miletus (5th century BC) is thought to have originated the atomic philosophy. His famous disciple, Democritus of Abdera, developed and named the building blocks of matter atomos, meaning literally indivisible, about 430 BC. Democritus believed that atoms were uniform, solid, hard, incompressible, and indestructible and that they moved in infinite numbers through empty space until stopped. Differences in atomic shape and size determined the various properties of matter. In Democritus' philosophy, atoms existed not only for matter but also for such qualities as perception and the human soul. For example, sourness was caused by needle-shaped atoms, while the colour white was composed of smooth-surfaced atoms. The atoms of the soul were considered to be particularly fine. Democritus developed his atomic philosophy as a middle ground between two opposing Greek theories about reality and the illusion of change. He argued that matter was subdivided into indivisible and immutable particles that created the appearance of change when they joined and separated from others. The philosopher Epicurus of Samos (341270 BC) used Democritus' ideas to try to quiet the fears of superstitious Greeks. According to Epicurus' materialistic philosophy, the entire universe was composed exclusively of atoms and void, and so even the gods were subject to natural laws. Most of what is known about the atomic philosophy of the early Greeks comes from Aristotle's attacks on it and from a long poem, De rerum natura (On the Nature of Things), which the Latin poet and philosopher Titus Lucretius Carus (c. 9555 BC) wrote to popularize its ideas. The Greek atomic theory is significant historically and philosophically, but it has no scientific value. It was not based on observations of nature, measurements, tests, or experiments. Instead, the Greeks used mathematics and reason almost exclusively when they wrote about physics. Like the later theologians of the Middle Ages, they wanted an all-encompassing theory to explain the universe, not merely a detailed experimental view of a tiny portion of it. Science constituted only one aspect of their broad philosophical system. Thus, Plato and Aristotle attacked Democritus' atomic theory on philosophical grounds rather than on scientific ones. Plato valued abstract ideas more than the physical world and rejected the notion that attributes such as goodness and beauty were mechanical manifestations of material atoms. Where Democritus believed that matter could not move through space without a vacuum and that light was the rapid movement of particles through a void, Aristotle rejected the existence of vacuums because he could not conceive of bodies falling equally fast through a void. Aristotle's conception prevailed in medieval Christian Europe; its science was based on revelation and reason, and the Roman Catholic theologians rejected Democritus as materialistic and atheistic.