Stable element
From Freepedia
In nuclear chemistry, the term stable element has been variously defined by different people at different times. Some observations follow.
The chemical element bismuth has been called the heaviest (highest atomic mass) stable element.
Some people consider elements like uranium, radium to be stable. They are radioactive and undergo radioactive decay to other elements. But they are isolatable from minerals mined from the earth's crust, in pure form, in bulk quantities. They can be formed into ingots and shipped or handled (with care) and have uses in industry. Some of these elements are even used in the home, without special precautions; for example, uranium oxide used to be used to glaze ceramic dishes, and thorium is found in lantern mantles. Bismuth is even used in medications. The term unstable element is in this case reserved for elements such as Roentgenium, which are never found in Earth's minerals. The status of elements such as Americium, Technetium, or Neptunium, which are not found naturally but are synthesised in bulk for industry remains dubious in this case.
Researchers at the Institut d'Astrophysique Spatiale in Orsay, France measured the alpha emission half-life of Bi-209 to be (1.9 ± 0.2) × 1019 years, meaning that bismuth is not stable after all. However, this half-life is on the order of one thousand million times the life span of the universe to date. Tungsten, used in household light bulbs, likewise has no stable isotopes, but does have isotopes with an exceptionally long half-life. Lead is now the heavest element which has isotopes which have not been shown to decay in the laboratory.
It has been noted, by some definitions of quantum physics that a proton may decay, on the order of 1040 years. By this criterion, no elements are stable. But the time span is several time the projected life span of the universe, and more than the square of the number of years that stars will exist.
In conclusion, the term stable element is more a definition of human perceptions than a defined physical property.
