 | Vocabulary |
allotrope atomic number atomic weight atoms Dalton's atomic theory isotope mixture nucleus substance |
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This modern definition resolves some sticky situations encountered with older definitions:
- The definition above doesn't make "element" and "mixture" mutually exclusive classifications of matter. A pure element sample that consists of several isotopes is actually a mixture- the isotopes can be separated by physical means. For example, mass spectrometry physically separates the isotopes of an element. Uranium-235 can be separated from uranium-238 using the fact that gaseous compounds of the two elements will spray from a pinhole into a vacuum at different rates.
- There isn't any problem with recognizing different chemical forms of an element (allotropes) as "elements". For example, oxygen can be found as O, as O2, or as O3 in different regions of the atmosphere. These three allotropes have different chemical behaviors and different chemical formulas- but all are samples of the element oxygen.
Obsolete 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.