Reality → Matter → Elements → Isotopes
Atoms of the same element that differ by the number of neutrons are called isotopes. The nucleus determines the mass, while the electron cloud determines chemical properties. Since isotopes of the same element have the same number of electrons and protons, their chemical and largely also physical properties are very similar; the atomic mass, however, varies [1] . More than 300 isotopes are known to occur naturally, of which about two thirds are considered to be stable and one third radioactive [2] . In addition, about 2000 artificial isotopes have been created in nuclear reactors and particle accelerators, all radioactive, generally with very short half-lives [3] . Some radioisotopes have important applications: in nuclear medicine, they are used for diagnostics, therapy, and research; and in science, the carbon C-14 isotope is being used to determine the age of organic remains in archaeological sites [4] .
The atomic mass of an element is therefore defined as the average mass of its isotopes weighted by their occurrence.
Radioactivity is caused by decay of the nucleus into lighter elements. An isotope is considered stable when no decay has ever been observed (e.g., by a scintillator). However, it is uncertain whether any truly stable isotopes do exist (e.g., the bismuth-209 isotope has long been considered to be stable, but in 2003 it was discovered that its half-life is more than 1019 years, billion times the age of the universe).
Some transuranium isotopes have half-lives in the order of a few seconds, but most have half-lives of only a fraction of a second (down to a few microseconds). Only very few of the 2000 known transuranium isotopes have half-lives of thousands or even millions of years (see Fission, Note 5, on the waste problem).
In the atmosphere, cosmic rays create the radioactive carbon-14 isotope (half-life 5,700 years) from the stable C-12 atom. The fraction of C-14 in atmospheric carbon is 1 part per trillion. The age of archaeological samples can be calculated from their lower fraction of C-14. The method works on samples up to about 60,000 years of age. For older sites, the age of rocks can be determined through several other radiometric dating methods. A common method is to measure the ratio of uranium and lead, the end product of uranium decay. That method, due to uranium's long half-life, can determine ages up to more than 2 billion years.