An isotope is a variant of an element that has the same number of protons in its nucleus but a different number of neutrons. Isotopes of the same element have the same atomic number but different atomic masses.
In radiometric dating, isotopes are used to determine the age of rocks and fossils based on their radioactive decay. Some isotopes are radioactive, meaning that they are unstable and decay over time, releasing energy and particles from their nucleus. This decay occurs at a constant rate, known as the half-life, which is the time it takes for half of the original radioactive atoms to decay.
By measuring the ratio of parent isotopes (the original, radioactive isotopes) to daughter isotopes (the decay products) in a rock or fossil, scientists can determine how much time has passed since the rock or fossil was formed. This is because the ratio of parent to daughter isotopes changes over time as the parent isotopes decay into daughter isotopes. The longer the time since the rock or fossil was formed, the more daughter isotopes will be present and the higher the ratio of daughter to parent isotopes will be.
Different isotopes have different half-lives, which means that they are useful for dating materials of different ages. For example, carbon-14 is a useful isotope for dating organic materials up to about 50,000 years old, while uranium-238 is useful for dating rocks that are billions of years old. By using a combination of different isotopes with different half-lives, scientists can determine the age of materials spanning a wide range of geological and historical timescales.