Radioactive decay and dating
" Another way of explaining it is that when geologists talk about isotopes, they are talking about one element of differing masses.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).
Radioactivity and radioactive decay are spontaneous processes.In reality, every atom is an isotope of one element or another.However, we generally refer to isotopes of a particular element (e.g., Rubidium-87 (Pb)).There's a small amount of radioactive carbon-14 in all living organisms.When they die no new carbon-14 is taken in by the dead organism.Radioactive decay involves the spontaneous transformation of one element into another.
The only way that this can happen is by changing the number of protons in the nucleus (an element is defined by its number of protons).
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.
The carbon-14 it contained at the time of death decays over a long period of time.
By measuring the amount of carbon-14 left in dead organic material the approximate time since it died can be worked out.
Often students get bogged down in the fact that they don't "understand" how and why radioactive elements decay and miss the whole point of this exercise.