Dalton's atomic theory
This modern definition resolves some sticky situations encountered with older definitions:
An element is a substance that cannot be decomposed into simpler substances.
This was the first useful definition of element, because it allowed elements to be distinguished from other substances experimentally. If a material could be broken down into two or more new substances which, when recombined, formed the original material, the material was not an element. Of course, it's impossible to use this definition to prove that a material actually is an element. A substance which couldn't be decomposed in one century could be decomposed in the next, when more advanced separation technologies became available.
The discovery of isotopes made the definition of elements as undecomposable substances untenable. Elements can be decomposed into isotopes, which have very slightly different properties than the original element sample. Recombining the isotopes gives back the original sample, so by this definition, any element that consists of more than one isotope is not a true element.
An element is a substance composed of identical atoms.
This definition was one of the cornerstones of John Dalton's atomic theory, but it too was made obsolete by the discovery of isotopes. Atoms of the same element have different numbers of neutrons in their nuclei. That means that atoms of the same element can also vary in mass.
Dalton's own atomic weight determinations seem not to support the contention that atomic mass rather than atomic number is a distinguishing characteristic for elements; his experimental data implied that several different metallic elements had the same atomic weight.
Author: Fred Senese firstname.lastname@example.org
Copyright © 1997-2010 by Fred Senese
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/matter/faq/print-what-is-an-element.shtml