As a result it is always undergoing natural radioactive decay while the abundances of the other isotopes are unchanged.

Carbon-14 is most abundant in atmospheric carbon dioxide because it is constantly being produced by collisions between nitrogen atoms and cosmic rays at the upper limits of the atmosphere.

Some chemical elements have more than one type of atom. Carbon has two stable, nonradioactive isotopes: carbon-12 (12C), and carbon-13 (13C).

Signals of this kind are often used by chemists studying natural environments.

A hydrocarbon found in beach sediments, for example, might derive from an oil spill or from waxes produced by plants.

Carbon-14 has a relatively short half-life of 5730 years, meaning that the amount of carbon-14 in a sample is halved over the course of 5730 years due to radioactive decay.

By about ten half-lives, or 58,000 years, the amount of carbon-14 left in the fossil is very little- about 1/1000 of the original number of carbon-14 atoms in the fossil.

Along with hydrogen, nitrogen, oxygen, phosphorus, and sulfur, carbon is a building block of biochemical molecules ranging from fats, proteins, and carbohydrates to active substances such as hormones.

All carbon atoms have a nucleus containing six protons.

The amount of carbon-14 gradually decreases through radioactive beta decay with a half-life of 5,730 years.

So, scientists can estimate the age of the fossil by looking at the level of decay in its radioactive carbon.

The method was developed by Willard Libby in the late 1940s and soon became a standard tool for archaeologists.