Atomic theory

atomic theory   n.  The physical theory of the structure, properties, and behavior of the atom.

Main Entry: atomic theory
Function: noun
1 : a theory of the nature of matter: all material substances are composed of minute particles or atoms of acomparatively small number of kinds and all the atoms of the same kind are uniform in size, weight, and other properties called also atomic hypothesis
2 : any of severaltheories of the structure of the atom; especially : one based on experimentation and theoretical considerations holding that the atom is composed essentially of a small positivelycharged comparatively heavy nucleus surrounded by a comparatively large arrangement of electrons

atomic theory

ancient philosophical speculation that all things can be accounted for by innumerable combinations of hard, small, indivisible particles (called atoms) of various sizes but of the same basic material; or the modern scientific theory of matter according to which the chemical elements that combine to form the great variety of substances consist themselves of aggregations of similar subunits (atoms) possessing nuclear and electron substructure characteristic of each element. The ancient atomic theory was proposed in the 5th century BC by the Greek philosophers Leucippus and Democritus and was revived in the 1st century BC by the Roman philosopher and poet Lucretius. The modern atomic theory, which has undergone continuous refinement, began to flourish at the beginning of the 19th century with the work of the English chemist John Dalton. The experiments of the British physicist Ernest Rutherford in the early 20th century on the scattering of alpha particles from a thin gold foil established the Rutherford atomic model of an atom as consisting of a central, positively charged nucleus containing nearly all the mass and surrounded by a cloud of negatively charged planetlike electrons.

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