These radioactive isotopes are unstable, decaying over time at a predictable rate.As the isotopes decay, they give off particles from their nucleus and become a different isotope.
If your mom walked in and saw 1/16th of a pie (and knew that you had been eating half of what was left every day), she could quickly determine that the pie was four days old. Some elements (uranium, for example) are unstable, and decay to a different element (uranium to lead, for example).You should keep this in mind when we are talking about determining the age of rocks.A more relevant example: Let us say that a rock is sitting on the surface of the Moon.Levels of carbon-14 become difficult to measure and compare after about 50,000 years (between 8 and 9 half lives; where 1% of the original carbon-14 would remain undecayed).The question should be whether or not carbon-14 can be used to date any artifacts at all? There are a few categories of artifacts that can be dated using carbon-14; however, they cannot be more 50,000 years old.
Thus, if we start out with 1 gram of the parent isotope, after the passage of 1 half-life there will be 0.5 gram of the parent isotope left.