Back in the 1940s, the American chemist Willard Libby used this fact to determine the ages of organisms long dead.
Most carbon atoms have six protons and six neutrons in their nuclei and are called carbon 12. But a tiny percentage of carbon is made of carbon 14, or radiocarbon, which has six protons and eight neutrons and is not stable: half of any sample of it decays into other atoms after 5,700 years.
Carbon 14 has a half-life of 5,780 years, and is continuously created in Earth's atmosphere through the interaction of nitrogen and gamma rays from outer space.
a nonmetallic element existing in the three crystalline forms: graphite, diamond, and buckminsterfullerene: occurring in carbon dioxide, coal, oil, and all organic compounds.
The isotope carbon-12 has been adopted as the standard for atomic wt.; carbon-14, a radioisotope with a half-life of 5700 years, is used in radiocarbon dating and as a tracer.
Once an organism is dead, however, no new carbon is actively absorbed by its tissues, and its carbon 14 gradually decays.
Libby thus reasoned that by measuring carbon 14 levels in the remains of an organism that died long ago, one could estimate the time of its death.
Symbol: C; atomic no.: 6; atomic wt.: 12.011; valency: 2, 3, or 4; relative density: 1.8--2.1 (amorphous), 1.9--2.3 (graphite), 3.15--3.53 (diamond); sublimes at 3367±25°C; boiling pt.: 4827°circulation of carbon between living organisms and their surroundings.