Artificial element
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An artifical element is an element that has no stable isotopes and is not naturally found on Earth.
Artifical elements are distinguised from radioactive elements by the fact that the artifical elements do not occur in appreciable quantities in the Earth's crust or atmosphere. Technetium is an example of an artificial element because only minute traces occur naturally in the Earth's crust (as a spontaneous fission product of uranium or by neutron capture in molybdenum ores), but technetium is found naturally in red giant stars. Uranium and thorium have no stable isotopes, but are found naturally in the Earth's crust and atmosphere.
Atomic mass for natural elements is based on weighted average abundance of natural isotopes occurring in the Earth's crust and atmosphere. For artificial elements the isotope formed depends on the means of synthesis, so the concept of natural isotope abundance has no meaning. Therefore for artificial elements the total nucleon (protons plus neutrons) count of the most stable isotope (ie, the isotope with the longest half-life) is listed in brackets as the atomic mass.
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