What makes radioisotopes useful for dating objects?
radioactive isotopes have a definite half life that is constant.
The percentage of the isotope left allows a calculate of age.
The calculation of the age is dependent on assuming that the sample starts with 100% of the isotope. Then knowing the half life of the isotope, the age of the sample can be calculate from the percentage of the isotope remaining.
For example Carbon 14 has a half life of approximately 5,600 years.
It is reasonable to assume that when a tree dies it has 100% of the level of Carbon 14 in the atmosphere at that time. If a piece of wood has 50% of the present level of Carbon 14, then one half life of the isotope has pasted and the age of wood is 5,600 years. If the wood has only 25 % of present level of Carbon 14 then two half lives have past and the wood is 11,200 years old.
These types of calculations can be done for any percentage of carbon 14 left in the wood. The accuracy of these calculation decreases as the percentage of the isotope left decreases. after four half lives only 1/16 or 5.5% of the isotope is left giving an age of
21,600 years. After seven half lives (37,200 years) there is less than 1% left (.6875% ). This makes any age greater than 40,000 years suspect.
The calculation of ages based on isometric dating start with an assumption of starting with 100% of the isotope, not losing any of the isotope during the time being measured, and an accurate determination of the percent of the isotope left.