Explain the method of radio-carbon dating to determine the age of plant and animal fossils.
Carbon in nature is found in different isotopes, meaning that the number of neutrons in the nucleus can differ. The most common carbon isotope found in nature is called C-12, meaning that the nucleus contains 6 protons and 6 neutrons. But <1% of all naturally occuring carbon is the C-14 isotope, meaning that the nucleus has 6 protons and 8 neutrons. The C-13 isotope is naturally mildly radioactive and beta decays at a constant rate (called the half life). Once an organism dies it no longer accumulates any additional C-14 in its body and the radioactive decay begins to occur at a steady rate. So the amount of C-14 can be measured and then based on the decay rate you can count back in time to when the organism died. Keep in mind that there is a margin of error here so radiocarbon dating will give you date range and not really an exact year.