Expert: Richard J. Raridon - 6/21/2005

Question
If a scientist purifies 1.0 gram of radium-226, how many years must pass before
only 0.50 gram of the original radium-226 sample remains unchanged?
The answer is 1600, but how do we know from the original question that when it says original, it doesn't mean before the purification.ANd then how would you be abe to solve the problem.They don't give you the original amount of uranium?

Answer
The half-life of Ra-226 is 1600 years.  The definition of half-life is the time for one half of a given number of atoms to undergo a change by emitting a particle and going to a lower energy level.  Half-lives have been measured for many radioactive atoms and are listed in tables.  It doesn't have to be purified as long as you specify only
Ra-226 atoms.  When Ra-226 emits an alpha particle, the remaining atom is Rn-222, radon, which you may have heard of.