I was recently doing some physics tuition on radioactivity and the student claimed her chemistry teacher had said that radioactive substances can be grouped into two divisions: those whose half-life is constant and those whose half-life changes over time.
I had never heard of this before and can't think of any reason why a half-life should change, so does anyone else know anything about this?
(I know some half-lives can be altered under certain conditions, but I'm talking about a natural change over time).
Best Answer
The short answer is no: halflives are constant.
However, let's discuss a situation in which that comment might have some kind of truth behind it. If you have a parent nucleus that decays to a radioactive daughter so that there will be two (or more) decays before stability. In general there are two possibilities for this:
The latter case is interesting to us here because at the start the sample will register an activity that decays with the parent's (short) halflife, but after a number of those halflifes have passed the activity of the sample will be dominated by the daughter and exhibit a longer halflife.
That is something that your instructor could have meant which would not be wrong. However, the halflife of each isotope remains the same: it is only the halflife of the sample (which contains more than one isotope) that varies.