The half-lives of several radioactive isotopes are known and are used often to figure out the age of newly found fossils.
Different isotopes have different half-lives and sometimes more than one present isotope can be used to get an even more specific age of a fossil.
If you have looked at a periodic table, you may have noticed that the atomic mass of an element is rarely an even number. If you are an atom with an extra electron, it's no big deal.
Half-life is defined as the time it takes for one-half of a radioactive element to decay into a daughter isotope.
Each original isotope, called the parent, gradually decays to form a new isotope, called the daughter.
Each isotope is identified with what is called a ‘mass number’.
Let's say you have 100g of uranium (don't try this at home, it’s radioactive). The half-life of uranium-238 is 4,500,000,000 years.
When 50g remain (and 50g have become something different), the amount of time that has passed is the half-life. That is a long time to wait for radioactive atoms to change, and many of the things that the original atoms change into are ALSO radioactive and dangerous!