See also: Molecule, Atomic Weight
The smallest unit of Chemical Elements (see Element) having the properties of that element. An atom contains several kinds of
particles. Its central core, the nucleus, consists of positively charged
particles, called Protons, and uncharged particles, called Neutrons. Surrounding the nucleus and orbiting it are negatively charged particles,
called Electrons. Each atom has an equal number of protons and electrons. The nucleus
occupies only a tiny fraction of an atom's volume but contains almost all of its Mass. Electrons in the outermost orbits determine the atom's chemical and
electrical properties. The number of protons in an atom's nucleus is called the Atomic Number. All atoms of an element have the same atomic number and differ in atomic
number from atoms of other elements (see Chemical Elements). The total number of protons and neutrons combined is the atom's Mass Number. Atoms containing the same number of proto
ns but different numbers of
neutrons are different forms, or Isotopes, of the same element.
History
In the 5th cent. B.C. the Greek philosophers Democritus and Leucippus proposed that Matter was made up of tiny, indivisible particles in constant Motion. Aristotle, however, did not accept the theory, and it was ignored for centuries. Modern
atomic theory began with the publication in 1808 by Dalton, John of his experimental conclusions that all atoms of an element have exactly the
same size and Weight, and that atoms of elements unite chemically in simple numerical ratios to
form Compounds. In 1911 Rutherford, Ernest explained an atom's structure in terms of a positively charged nucleus
surrounded by negatively charged electrons orbiting around it. In 1911 Bohr, Niels used Quantum Theory to explain why electrons could remain in certain allowed orbits without
radiating Energy. The development of quantum mechanics during the 1920s resulted in a
satisfactory explanation of all phenomena related to the role of electrons in atoms and
of all aspects of their associated spectra (see Spectrum). The quantum theory has shown that all particles have certain Wave properties. As a result, electrons in an atom cannot be pictured as lo
calized
in space but rather should be viewed as a cloud of Charge spread out over the entire orbit. The electron clouds around the nucleus
represent regions in which the electrons are most likely to be found. Physicists
are currently studying the behavior of large groups of atoms (see Solid-State Physics), and the nature of and relations among the hundreds of Elementary Particles, Subatomic Particles that have been discovered in addition to the proton, neutron, and electron.