Example of radiometric dating
Radioactive elements "decay" (that is, change into other elements) by "half lives." If a half life is equal to one year, then one half of the radioactive element will have decayed in the first year after the mineral was formed; one half of the remainder will decay in the next year (leaving one-fourth remaining), and so forth.The formula for the fraction remaining is one-half raised to the power given by the number of years divided by the half-life (in other words raised to a power equal to the number of half-lives).The rules are the same in all cases; the assumptions are different for each method.To explain those rules, I'll need to talk about some basic atomic physics. Hydrogen-1's nucleus consists of only a single proton.
Since there is now only 1/4 of the original amount of Parentium-123, we know that two half-lives of Parentium-123 have elapsed.
Radiometric dating is a means of determining the "age" of a mineral specimen by determining the relative amounts present of certain radioactive elements.
By "age" we mean the elapsed time from when the mineral specimen was formed.
I found several good sources, but none that seemed both complete enough to stand alone and simple enough for a What is radiometric dating?
Simply stated, radiometric dating is a way of determining the age of a sample of material using the decay rates of radio-active nuclides to provide a 'clock.' It relies on three basic rules, plus a couple of critical assumptions.