Isotopes - (n.)
"Isotopes" are atoms of the same element that have different atomic masses. Radioactive isotopes emit radiation and change to other atoms while stable isotopes do not change through time. The simplest of the elements, hydrogen, has three isotopes: normal hydrogen, deuterium, and tritium. Normal hydrogen consists of one proton and one electron. Deuterium ("heavy hydrogen", which makes "heavy water" with oxygen) has a neutron in addition to one proton and one electron, for an atomic mass of 2. It is stable. Tritium has yet another neutron, for an atomic mass of 3, and is radioactive with a half life of 12.3 years. It decays into helium-3 by emitting beta radiation (which changes a neutron to a proton). The word "isotope" means "same place," referring to the fact that isotopes of a given element have the same atomic number and hence occupy the same place in the Periodic Table. Thus, they are very similar in their chemical behavior. This similarity - and the subtle differences in behavior that arise from the differences in atomic mass - make isotopes useful as tracers of climate-related processes in atmosphere, ocean and biosphere, as well as in the reconstruction of climate history. Such history is contained in tree-rings, corals, mountain glaciers, polar ice, sedimentary deposits in lakes and on the seafloor, and a host of other geological repositories (e.g. soils, loess sequences, cave deposits, pack rat middens, evaporite and reef accumulations).