Dalton's Theory
   John Dalton, a British schoolmaster and chemist, was fascinated by the patchwork puzzle of the elements. Early in the 19th century he made studies of the way in which the various elements combine with one another to form chemical compounds. Although many other scientists, from the Greeks onward, had already speculated that the smallest units of a substance are atoms, Dalton is regarded as one of the most significant figures in atomic theory because he made the subject quantitative. He showed how these atoms link together in definite proportions. Subsequent investigations proved that atoms normally form groups called molecules. Each molecule of water, for example, consists of a single atom of oxygen and two atoms of hydrogen joined by an electrical force called a chemical bond. Water is symbolized as HOH, or as H2O, meaning that its molecule consists of two atoms of hydrogen joined to one atom of oxygen.

   All atoms of any given element behave in the same way chemically. Thus, from a chemical viewpoint, the atom is the smallest entity to be considered. The chemical properties of the various elements are quite different; their atoms combine in many different ways to form a multitude of different chemical compounds. Some elements, such as the noble gases helium and argon, are inert-that is, they fail to react with other elements except under special conditions. Unlike oxygen, which has a diatomic molecule (two atoms combined in a single molecule), helium and other inert gases are monatomic elements, with a single atom per molecule.

The Rutherford Nuclear Atom
   Recognition of the nature of radioactive emissions enabled physicists to penetrate more deeply into the atom, which was found to consist mostly of space. At the center of this space is a core called the nucleus, measuring only about a ten-thousandth of the diameter of the atom. Rutherford established that the mass of the atom is concentrated in its nucleus. He also proposed that the electrons, already known to form part of the atom, travel in orbits around the nucleus. The nucleus has a positive electrical charge; the electrons each have a negative charge.

   The charges carried by the electrons add up to the same amount of electricity as those residing in the nucleus, and thus the normal electrical state of the atom is neutral.

Bohr and his theory
   Bohr, Niels Henrik David (1885-1962), Danish physicist and Nobel laureate, who made basic contributions to nuclear physics and the understanding of atomic structure.

   Bohr was born in Copenhagen on October 7, 1885, the son of a physiology professor, and was educated at the University of Copenhagen, where he earned his doctorate in 1911. That same year he went to the University of Cambridge in England to study nuclear physics under J. J. Thomson, but he soon moved to the University of Manchester to work with Ernest Rutherford.

   Bohr's theory of atomic structure, for which he received the Nobel Prize for Physics in 1922, was published in papers between 1913 and 1915. His work drew on Rutherford's nuclear model of the atom, in which the atom is seen as a compact nucleus surrounded by a swarm of much lighter electrons. Bohr's atomic model made use of quantum theory and the Planck constant (the ratio between quantum size and radiation frequency). The model posits that an atom emits electromagnetic radiation only when an electron in the atom jumps from one quantum level to another. This model contributed enormously to future developments of theoretical atomic physics.

[Contact us] [Our resources] [Feedback] [Terms of use]