Carbon-14
Carbon-14,6-14-C, or radiocarbon, is a radioactive isotope of carbon with a nucleus that has 6 protons as every carbon atom in the universe but has 8 neutrons and outside it has 6 electrons normally because it can lose or gain electrons due to ionic or covalent bonding. (Protons don’t change in an atom because that would transform the element). Carbon-14 was discovered on the 27 of February of 1940 by Martin Kamen and Sam Ruben at the University of California Radiation Laboratory. And Franz Kurie had already suggested its existence in 1934. It is also very rare due to the fact that there is only one carbon 14 atom for every 3 billion carbon atoms in existence. (http://global.britannica.com/EBchecked/topic/94839/carbon-14-dating)
Carbon-14 is made in the higher layers of the atmosphere by neutrons absorbed by nitrogen atoms. So carbon-14 is made or produced at heights between 9 to 15 km
For example if an archeologist finds a bone from an unknown dinosaur the archeologist uses carbon 14 and carbon dating to find what era it belonged to and places it in the time it belongs to. This method was invented by Willard Libby in 1940. It works by measuring the amount of carbon 14 in the body and every living thing has carbon 14 because when it is produced in the atmosphere it comes down and plant get it when they do photosynthesis and then the animals eat the plant and get the carbon 14 and then the food chain comes in and every animal is eaten by its predator and after the top of the food chain gats carbon 14 every animal has it, even us humans have carbon 14 in our bodies. Radio carbon dating can work for even 50000 years ago but if you want to go further then you have to do some special preparations. .,