Expert: Florence M Rollwagen - 8/23/2008

Question
How can we determine the age of those fossils which are quite old and their radioactive isotopes have been completely emitted?

Answer
Hi Pooja:  Thanks for your question.

Most of the time when scientists analyze the age of rocks (containing fossils), they use more than one radioisotope.  This provides a check on each method.

Some isotopes decay very fast (seconds), some longer (thousand of years) and some very long indeed (billions of years).

The longest decay rate is that of uranium-235 decay to lead-206, which takes about 4.5 billion years.

If you need a refresher on radioisotope decay and the Periodic Table of the Elements, to go:
http://ie.lbl.gov/education/isotopes.htm

The various isotopes relate to the structure of the atom.  Atoms have protons and neutrons in their nuclei, with electrons located in a cloud circling the nucleus.  Each element has a fixed number of positively charged protons in the nucleus.  The number of neutrons can change, however, giving the various numbers that you saw above.  For example, Lead (Pb) can exist in 46 different isotopes, each with its own half life.

So, in answer to your question, if half the amount of U235 decays in 4.5 billion years, it would take a really long time before there were no more isotopes around.  There are no fossils older than the age of the earth, so the answer to your question is that there are no fossils in which all the isotope has decayed.

Here is more information on radiometric dating:

http://www.youtube.com/watch?v=YpJH6cLPcvI&feature=related

http://en.wikipedia.org/wiki/Radiometric_dating

I hope this answer has helped you.  Please write back if you have more questions.

FM Rollwagen, PhD