These two uranium isotopes decay at different rates. The half-life of the uranium-238 to lead-206 is 4.47 billion years.The uranium-235 to lead-207 decay series is marked by a half-life of 704 million years.With rubidium-strontium dating, we see that rubidium-87 decays into strontium-87 with a half-life of 50 billion years.By anyone's standards, 50 billion years is a long time.Because plants use carbon dioxide for photosynthesis, this isotope ends up inside the plant, and because animals eat plants, they get some as well.When a plant or an animal dies, it stops taking in carbon-14.In fact, this form of dating has been used to date the age of rocks brought back to Earth from the moon.So, we see there are a number of different methods for dating rocks and other non-living things, but what if our sample is organic in nature?
So, we rely on radiometric dating to calculate their ages.
So, if you know the radioactive isotope found in a substance and the isotope's half-life, you can calculate the age of the substance. Well, a simple explanation is that it is the time required for a quantity to fall to half of its starting value.
So, you might say that the 'full-life' of a radioactive isotope ends when it has given off all of its radiation and reaches a point of being non-radioactive.
For example, uranium-lead dating can be used to find the age of a uranium-containing mineral.
It works because we know the fixed radioactive decay rates of uranium-238, which decays to lead-206, and for uranium-235, which decays to lead-207.
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