Radiometric Dating. The most widely known is radiocarbon dating (or just carbon dating). All life on Earth is carbon based. There are many variants of carbon called "isotopes". For example carbons atomic number is 6, so though the most commonly identified carbon atom would have 6 protons and 6 neutrons, other isotopes carbon would have a different number of neutrons. In this fashion we look at two isotopes carbon-12 and carbon 14. 12 is stable and will remain unchanged. 14 has a half life of 5700 years. That is, the stored energy decays at a known amount. So carbon dating, uses the ratio of carbon 14's decay - versus the stable carbon 12 - establishing a date of fossilized organic material (animals or plants) up to around 60,000 years old. Anything older than that requires a different radiometric process involving different elements. Potassium-40 for example has a half life of 1.3 billion years.