MadSci Network: Physics

Re: How come element like uranium have such a long half-life?

Date: Tue Dec 8 19:49:34 1998
Posted By: Richard Bersin, Other (pls. specify below), Senior Technical Staff Member, Emergent Technologies
Area of science: Physics
ID: 912497689.Ph

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 
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....

Current Queue | Current Queue for Physics | Physics archives

Try the links in the MadSci Library for more information on Physics.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-1998. All rights reserved.