Chemistry / Unit 12 Chem 2530 worked example, 60 to 90 seconds
Radiometric Dating from a Decay Ratio
The idea
Radiometric dating means comparing a sample's present ratio of a radioactive isotope to its stable decay product, or to a stable reference isotope, against the ratio expected at the sample's formation, and solving the first-order decay relationship for elapsed time using the isotope's known half-life.
In practice
If a sample's carbon-14 activity has fallen to 79% of the activity expected in freshly formed organic matter, solving the first-order relationship with carbon-14's 5730-year half-life gives an elapsed time of roughly 1950 years since the organism died.
Where people slip
This method assumes the starting ratio at formation is known or can be reasonably estimated — applying it to a sample whose initial isotope ratio is not actually comparable to the assumed baseline produces a confidently wrong date.
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