Whereas the physical characteristics and properties of an element are determined by the number of protons in the atoms nucleus, the isotope of an element is determined by the number of neutrons in the nucleus of the atom. All elements by default have at least one isotope and most have several isotopes. For example, the simplest element, hydrogen, has three isotopes. All isotopes of hydrogen have one proton and one electron. The most common form or isotope of hydrogen, protium, has no neutrons whereas deuterium(symbol=D) has one (Hydrogen-2 or H-2) and tritium(T or H-3) has two.
Deuterium oxide, (D2O) [ also known as heavy water] is a stable isotope of hydrogen which is used as a cooling medium and neutron absorbing moderator in nuclear reactors. When an atom of deuterium absorbs a neutron it becomes tritium. Unlike deuterium, which is a stable isotope, tritium is unstable and radioactive. The radioactivity of the tritium isotope is very useful. When an atom of tritium decays, it emits a beta particle (electron) and antineutrino (proto-proton) leaving two protons and one neutron in the atom's nucleus. The remaining atom is no longer a hydrogen isotope but instead a new element, Helium, or to be more precise an isotope of helium called He-3.
Beta radiation is relatively harmless to humans as beta particles are absorbed by the dead outer layers of the skin. If ingested however, tritium would be considered toxic, but otherwise is pretty harmless stuff. The radiation it puts off will cause phosphorus or other luminescent chemicals to glow. Today it has replaced radium as a material to cause watch dial markings and exit signs to glow in the dark. It is also used in night vision scopes to provide illumination. The most common isotope of radium is an alpha particle emitter, but also emits hazardous gamma radiation. An alpha particle is a nucleon containing two protons and two neutrons, essentially a helium atom nucleus, and is generally not hazardous. Gamma radiation on the other hand, can cause substantial damage to living tissues. Soon after its discovery, exposure to radium was found to cause pernicious anemia, and even prolonged exposure to the small amount of radium once used to illuminate a watch dial was linked to development of bone cancer. Today, the best watches use tritium encased in tiny phosphor coated vials to light them up.
A final use of the isotope tritium which should not escape mention here, is its use as a substance to promote fusion in a hydrogen bomb. In this application, tritium gas under high pressure is further compressed and bombarded by neutrons from an atomic explosion. The result is that the strong nuclear force takes effect, fusing tritium atoms into helium and in the process releasing a tremendous amount of energy.
As noted already, unstable isotopes of elements like tritium decay over time becoming a different isotope or even a different element. The rate of this decay can be a very useful tool in determining the age of things. For instance, tritium has a half life of 12.3 years, which means that in that period one half of a given quantity of tritium will decompose to become helium-3. By measuring the ratio of tritium to H-3 in a given substance the age of the material can be determined within a year or so, perhaps eve a few months. The problem is, tritium's half life is too short to be useful and it is also a very rarified gas hear on Earth because it doesn't last very long. But there are some other elemental isotopes that work well for radio dating applications.
Carbon is an element with 3 isotopes. The normal stable isotope of carbon contains 6 protons and 6 neutrons and is therefore referred to as carbon-12 (C-12). The C-13 isotope is a stable isotope with 7 neutrons, but finds use in analytical chemistry. Carbon-14 which has 8 neutrons is unstable and therefore radioactive, having a half life of 5,730 years, give or take 40 years. Like tritium, carbon-14 experiences beta decay giving up an electron and antineutrino particle resulting in the stable non-radioactive isotope Nitrogen-14.
In the nineteen 50's and 60's, the percentage of C-14 in carbon dioxide molecules in Earths atmosphere was significantly increased due to testing of atomic devices. After the above ground nuclear test band treaty went into effect in 1963, the percentage of C-14 in the biosphere began to drop significantly. So marked was the rate of decline that scientist found they could accurately predict the age of people born in the northern hemisphere within a year to a year and a half by the amount of C-14 in their tooth enamel. This discovery led to extensive use of C-14 in archaeological investigations, permitting dating of carbon containing materials up to 50,000 years old with an error factor of 200 years. The use of radioactive isotopes to determine age evolved into a whole new wing of science known as radiometric dating. Today using the decay rates of uranium-238 (U-238) or U-235 into lead, scientist can data geological strata to within a million years over the past 4 billion years of Earths history.
But the isotope U-235 originally had a much more profound use than being used to date things on Earth. It turns out that U-235 and Plutonium-239 are two, and the only two elemental isotopes which possess a very unique property, they are fissionable. Furthermore, when a specific amount, known as a critical mass of either of these elements is achieved, the fission reaction becomes self perpetuating and the result is a nuclear explosion, an atomic bomb. This capability was first demonstrated on July 16, 1945 at the White Sands test range near Alamagordo N.M. In that moment the world, at least as far as human beings were concerned radically changed.
Today radioactive isotopes have found a wide range of uses beyond just their destructive capabilities and luminescent qualities. In the medical field, radio isotopes are used in x-ray machines, and as therapeutic medicines. For instance, a radioisotope of iodine is used for imaging tests or to treat an overactive thyroid, thyroid cancer, and certain other cancers. (http://www.cancer.gov/dictionary) There are actually a large number of isotopes that have found specific medical uses.
For years public water supplies were treated with chlorine and fluorine to destroy harmful bacteria. Unfortunately, after forty years of such treatments, it was found that these chemicals were also causing other unhealthy side effects to humans and animals consuming treated water. In example, Chorine can cause kidney damage in some people and even contribute to the onset of diabetes. Today, many water treatment plants add radioisotopes to water coming into the treatment plant which essentially sterilizes or kills all the bacteria. Another chemical is then added to cause the radioisotope to coagulate and both chemicals precipitate out. The added chemicals are removed by filtration and reclaimed, making the process both effective in providing ultra clean drinking water and cost efficient as well.
Just as with water, radioactive isotopes are used these days to irradiate food and kill and remove unwanted bacteria. This technique allows food to be shipped over greater distances and durations and still arrive at the store with a delicious fresh looking exterior. Meats like pork which often harbor parasites are irradiated with the isotope Cobalt-60 to kill the parasitic organisms. The isotope itself never comes in contact with the meat, but the gamma radiation produced through its decay penetrates all the way through meat and bone as well, killing any living organisms. In the same way, radioactive isotopes are regularly used as a pesticide. While other toxic chemical pesticides may not penetrate deep into the walls of your home to kill termites and other pests, radiation produced by radioisotopes, can and will, eliminate the pest problem without leaving behind a chemical residue that could be harmful to the homes occupants.
As mankind has learned how to safely and effectively use stable and unstable isotopes, these variations on elemental nucleon arrangements have provided many benefits contributing to the survival of the human species. Ironically, radioactive isotopes have also provided the potential of total human annihilation through weapons of mass destruction. We are left with only the hope that members of our species around the world, will continue to use isotopes intelligently and for the betterment of all life forms on the planet.
Radium hazards - http://www.epa.gov/radiation/radionuclides/radium.html
Food Irradiation - http://www.chem.duke.edu/~jds/cruise_chem/nuclear/food.html