Radioactive clocks
The radioactive decay rate is unaffected by physical and chemical conditions on Earth
Knowing (1) the half-life of a particular isotope
(2) the products into which it decays
(3) relative amount of parent and daughter atoms
- we can calculate how long it has been decaying !!! --> radioactive clock
Uranium-238 -- > Lead-206 (half-life of 4.5 billion years)
appropriate to date the lifetime of Earth and other
planets (Uranium is formed only during supernova,
- ratio of the stable 12C and radioactive 14C isotopes
- half-life for 14C: 5700 years
- during the life of an organism the 14C is replaced
through the metabolism 12C/14C is constant
- after the organism dies 14C is not anymore replaced
12C/14C increases! --> possibility to date the death
(not useful beyond 40,000 years) --> need other radioactive clocks