C 14 dating can be used on
So, at the time of death, the C14 : C12 ratio is fixed.C14 undergoes radioactive decay (into C12) so the C14 : C12 ratio declines and that can be used as a measure of the time since death. Carbon 14 (C14) is created in the upper atmosphere by the action of cosmic rays on Nitrogen atoms.C14 is continuously made in the atmosphere by the effects of solar radiation on Nitrogen14. Since its half-life is about 5000 years, it can be used a) to date items that contain a reasonable amount of carbon (especially remains of living beings), and b) up to a limit of about 50,000 years.
Carbon 14 can only be used to date fossils of a very recent age.
It is because living organisms absorb C14 from their environment.
After death, they are no longer capable of absorbing any more C14.
Obviously this circulation stops when a C14 molecule gets incorporated into a plant or something that ate the plant. This is a very useful isotope for measuring archaeological material, and with a good sample, one may date a sample to about ten half lives - about 57300 years. The uncertainty of the measurement increases with the total time. The atmosphere consists of a certain ratio of carbon 14 to carbon 12. If you add a neutron you get c13 which is also stable.
Over time the C14 in a tree disappears while C14 in the iar keeps gettin renewed. C14 is not useful for recent dating because of the quantity of material released during atomic bomb testing. Although this ratio has been in decline since the dawn of the industrial revolution, when humans began pumping depleted CO2 into the atmosphere (fossil carbon contains virtually no C14), the ratio is sufficiently constant that accurate ages can be obtained from any organic material within ten half lives of C14. However one more neutron gives you C14 which is unstable. C14 is a very useful element in determining the age of younger fossils, up to 60-70,000 years old. Radioisotopes decay at a rate specific to the particular element (and its isotope).