Radioactivity

Radioactivity is produced when the nucleus of an atom breaks apart or decays.

This radioactivity can be used to date rocks/fossils - absolute dating. Many radioactive materials, such as Uranium-235, decay at a constant rate.
This specific rate is half-life - the time it takes for 1/2 (50%) of the material to decay into a stable substance/isotope. Remember an isotope is the same element but with a different number of neutrons. This change in the number of neutrons will change the mass of the atom

Let's look at an example using the graph above. If we assume we have 20 grams of the parent material, half-life of 5 years, how many grams will be left of the parent after one half-life and how much time has passed? After one half-life 50% or 1/2 (10 grams) of the parent material will be left and 5 years will have passed. After 2 half-lives have passed 25% or 1/4 (5 grams) of the parent material is left and 10 years have passed. Remember the time remains constant!

Uranium is great for dating real old stuff- millions of years but for recent geologic events (up to 75,000 years ago) radiocarbon is used, Carbon-14.