ATOMIC PHYSICS

the scientific study of the structure of the atom, its energy states, and its interactions with other particles and fields. Atomic physics has proved to be a spectacularly successful application of quantum mechanics, which is one of the cornerstones of modern physics. The notion that matter is made of fundamental building blocks dates to the ancient Greeks, who speculated that earth, wind, fire, and water might form the essence of the physical world. Little was done, however, to advance the idea that matter might be made of tiny particles until the 17th century. The English physicist Isaac Newton, in his Principia Mathematica (1687), proposed that Boyle's law, which states that the product of the pressure and the volume of a gas is constant at the same temperature, could be explained if one assumes that the gas is composed of particles. In 1808 the English physicist John Dalton suggested that each element consists of identical atoms, and in 1811 the Italian physicist Amedeo Avogadro hypothesized that the particles of elements may consist of two or more atoms stuck together. Avogadro called such conglomerations molecules, and based on experimental work he conjectured that the molecules in a gas of hydrogen or oxygen are formed from pairs of atoms. During the 19th century, there developed the idea of a limited number of elements, each consisting of a particular type of atom, that could combine in an almost limitless number of ways to form chemical compounds. At mid-century the kinetic theory of gases successfully attributed such phenomena as the pressure and viscosity of a gas to the motions of atomic and molecular particles. By 1895 the growing weight of chemical evidence and the success of the kinetic theory left little doubt that atoms and molecules were real. The internal structure of the atom, however, became clear only in the early 20th century, with the work of the British physicist Ernest Rutherford and his students. Until Rutherford's efforts, a popular model of the atom had been the so-called plum-pudding model, advocated by the English physicist J.J. Thomson, which held that each atom consists of a number of electrons (plums) embedded in a gel of positive charge (pudding); the total negative charge of the electrons exactly balances the total positive charge, yielding an atom that is electrically neutral. Rutherford conducted a series of scattering experiments that challenged Thomson's model. Rutherford observed that, when a beam of alpha particles (which are now known to be helium nuclei) struck a thin metal foil, some of the particles were deflected backward. Such large deflections were inconsistent with the plum-pudding model. Rutherford's work led to the modern understanding of the atom, in which a heavy nucleus of positive charge is surrounded by a cloud of light electrons. The nucleus is composed of positively charged protons and electrically neutral neutrons, each of which is approximately 2,000 times as massive as the electron. The atoms of each chemical element radiate a spectrum with distinctive wavelengths, which reflect the atomic structure. Because atoms are so minute, their properties must be inferred by indirect experimental techniques. Chief among these is spectroscopy, which is used to measure and interpret the electromagnetic radiation emitted or absorbed by atoms as they undergo transitions from one energy state to another. Through the procedures of wave mechanics, the energies and characteristic wavelengths of atoms in various energy states may be computed from certain basic atomic constants, namely, the electron mass and charge, the speed of light, and the fundamental characteristic constant of the quantum theory, Planck's constant. Based on these fundamental constants, the numerical predictions of atomic quantum theory can account for most of the observed properties of different atoms. In particular, quantum mechanics offers a deep understanding of the arrangement of elements in the periodic table, showing, for example, that elements in the same column of the table should have similar properties.