September 25, 2011 Leave a comment
By request, I shall use my expertise to explain the magic that is carbon dating – what allows archaeologists to deduce the age of ancient artifacts.
Just kidding, I’ll link you to someone better: How Stuff Works has a 1 minute video that explains it pretty concisely. I wish I could embed it here, but WordPress is a jerk like that.
If for some reason that video didn’t do the subject justice for you, here’s the deal, in some more detail:
Isotopes: An atom has protons, neutrons and electrons. The number of protons defines what element it is; for example, carbon has 6 protons, boron has 5, nitrogen has 7. The number of electrons (compared to the given number of protons) determines if it’s charged or not (an ion): if there are more electrons than protons, it’ll be negatively charged (an anion), and if there are less electrons than protons it’ll be positively charged (a cation).
Finally, the number of neutrons an atom has will determine what isotope of the element it is. Having more or less neutrons doesn’t make it a different element or make it charged, but it determines the atom’s nuclear behaviour – such as radioactivity (when an atom spontaneously shoots out particles).
Carbon: Most carbon in the atmosphere (in CO2) is the isotope carbon-12; that means there are 12 particles in the nucleus, in this case 6 protons and 6 neutrons. There is also the stable isotope carbon-13, and the radioactive isotope carbon-14, which is created when cosmic rays whack nitrogen-14 atoms, replacing a proton with a neutron. This transformation doesn’t last though; carbon-14 spontaneously decays into nitrogen-14, with a half-life of 5730 years.
Carbon-14 is very rare (compared to the other carbon isotopes) but is still everywhere, so it gets processed by organisms and incorporated into their structure (remember, all life is basically built out of carbon). If a carbon-14 atom decays into nitrogen while an organism is alive, the organism will just replace it with another carbon, but once an organism dies its carbon-14 content is slowly replaced by nitrogen-14.
Dating: Scientists can predict what the ratio of carbon-14 to carbon-12 was in the living organism (point A) based on the isotope ratio in the atmosphere, and they can compare that to the ratio they find in the dead organism now (point B). Since they know the rate at which carbon-14 decays, they can tell how long it took to get from point A to point B.
Ta da! Radiocarbon dating. There are a lot of assumptions involved in the above simplification, but I won’t attempt to go into any more detail; you can take a peek yourself on wikipedia. Note also that radiocarbon dating is just one example of radiometric dating, which can be used for older substances since other isotopes have longer half-lives than carbon-14.
Whew, I’m exhausted. I hope that helped 🙂 feel free to ask me if you have any remaining questions, or if there’s any other topic you’d like explained.