Many students have difficulty with the concept of half-life of radio-active isotopes. The following video shows how to model it with a bunch of dice. Try this with your students. https://www.youtube.com/watch?v=HRwey6cwGHo
The half-life of radio-active isotopes is given in the table found here: https://cds.cern.ch/record/1309915/files/978-3-642-02586-0_BookBackMatter.pdf
Note that each isotope has a specific half-life that does not change. However, the half-life of different isotopes varies from milliseconds to billions of years.
Please post questions.