MadSci Network: Physics |
Dear Paothep, I am going to answer your questions about uranium. But first you should understand a few things about radioactivity. Uranium 238 is a radioactive material. This means that its atoms spontaneously decay by giving off radiation, (alpha, beta, etc.). Each atom, however, is individual and separate from all of the other atoms. When one atom decays completely it will never decay again. It cannot give off any more radiation. Now a piece of pure uranium has many many atoms in it. For example a piece of uranium which weighs 238 grams will have 6X10^23 atoms of uranium in it. These atoms will all be radioactive, but they decay totally independently of each other. By measurements we have learned that in the span of 4,510,000,000 years one half of the atoms in this piece of uranium will decay but we will never know which one will decay next! They are all independent of each other. So when one atoms decays it is finished forever. But there are lots of atoms left to still decay. We know that 1/2 of the atoms will decay in the time period I indicated above. So, you are right that as the individual atoms decay there are fewer and fewer left that are the original radioactive species and so the number of atoms left to decay will slowly get smaller and smaller. But if it takes all that time for only 1/2 of them to decay then it will be a very long time before you could notice that there is less radioactivity. That time is called the half-life of uranium. Now there are some radioactive atoms that have a much shorter half life. For example, tritium is a radioactive isotope of hydrogen which has a 1/2 life of 12.32 years. So every 12.32 years 1/2 of the tritium atoms decay. In 100 years the tritium will be pretty much gone because so many halflives will have passed. Remember, however, that the half life of uranium is always the same and that it is just that there are fewer and fewer atoms left to decay. Remember we never know when any particular atom will decay (that's up to the laws of probability and statistics); we only know for sure that 1/2 of the atoms that are radioactive will decay in one half-life of the radioactive species. I hope that answers your question. Richard Bersin....
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