

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 onefourth remaining), and so forth.The formula for the fraction remaining is onehalf raised to the power given by the number of years divided by the halflife (in other words raised to a power equal to the number of halflives).The isotope potassium40 (k40) decays into a fixed ratio of calcium and argon (88.8 percent calcium, 11.2 percent argon).Since argon is a noble gas, it would have escaped the rockformation process, and therefore any argon in a rock sample should have been formed as a result of k40 decay.The sum of protons plus neutrons is the mass number.We designate a specific group of atoms by using the term "nuclide." A nuclide refers to a group of atoms with specified atomic number and mass number.


In all radiometric procedures there is a specific age range for when a technique can be used.Any argon present in a mineral containing potassium40 must have been formed as the result of radioactive decay.F, the fraction of K40 remaining, is equal to the amount of potassium40 in the sample, divided by the sum of potassium40 in the sample plus the calculated amount of potassium required to produce the amount of argon found. In spite of the fact that it is a gas, the argon is trapped in the mineral and can't escape.An atom with the same number of protons in the nucleus but a different number of neutrons is called an isotope.For example, uranium238 is an isotope of uranium235, because it has 3 more neutrons in the nucleus.If we knew the fraction of a radioactive element still remaining in a mineral, it would be a simple matter to calculate its age by the formula To determine the fraction still remaining, we must know both the amount now present and also the amount present when the mineral was formed.
