Radioactive dating exercises
The units of half-life are always time (seconds, minutes, years, etc.).
Isotopes of an element are atoms that all have the same atomic number (or number of protons in the nucleus) but have different atomic masses (hence different numbers of neutrons in the nucleus).
There are a number of ways that this can happen and when it does, the atom is forever changed.
There is no going back -- the process is irreversible. When we pour our popcorn kernels into a popcorn popper, the is no way to know which will pop first.
Students often struggle with this concept; therefore, it should be stressed that it is impossible to know exactly when each of the radioactive elements in a rock will decay.
Statistical probablity is the only thing we can know exactly.