How do scientists determine the age of an object using isotopes?

Answer 1

Scientists use the relative amount of stable and unstable isotopes in an object to determine its age.

Answer 2

Carbon-14 dating relies on the fact that due to incidence of cosmic rays on the earth's atmosphere, carbon-14 is constantly being made by reaction with nitrogen in the atmosphere. Now carbon-14 has a half life of about 5500 years, so an equilibrium level of carbon-14 is set up, and over a long period of time remains roughly constant. All living things absorb carbon from the atmosphere in carbon dioxide, but when they die this stops. Therefore at the time of death the ratio of carbon-14 to the stable varieties of carbon has a certain value which is basically considered to have been constant up to today, but after death the carbon-14 starts to decay with the above half life, so if the sample of the dead organic matter is analysed, the present ratio of carbon-14 to stable carbon gives an indication of how long the organic matter has been dead.

In actual practice the process is much more complicated and all sorts of corrections and calibrations have to be made, but that is the principle behind it.

Answer 3

Isotopes are in almost everything in very small amounts. Most isotopes are unstable so will release radiation because they break down into simpler elements to gain stability. By measuring the half-life of these isotopes and by comparing present amounts of the isotopes and the original amounts, they're able to determine the age.

In order for this to work, you need a reference. As an example, the best reference for radioactive dating is carbon dating, which measures the ratio of carbon-14 to carbon-12 and carbon-11 in a sample. This takes advantage of the fact that the carbon-14 ratio is relatively known by comparison with other dating techniques, and on the fact that the carbon-14 ratio is stable until the plant material containing it "dies". The "complexity" in this method is that the carbon-14 ratio was not constant - it varied over time - so compensation techniques are used.