Atmospheric carbon-14 rapidly reacts with oxygen in air to form carbon dioxide and enters the carbon cycle.

Carbon dating, also known as radiocarbon dating, is a method of estimating the age of carbon-bearing materials up to 60,000 years old.

Radioactive carbon-14 is continually formed in the atmosphere by the bombardment of cosmic ray neutrons on nitrogen-14 atoms.

After it forms, carbon-14 naturally decomposes, with a half-life of 5,730 years, through beta-particle decay.

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 half-life is always the same regardless of how many nuclei you have left, and this very useful property lies at the heart of radiocarbon dating. The graph below shows the decay curve (you may recognize it as an exponential decay) and it shows the amount, or percent, of carbon-14 remaining.

You will notice that after around 40,000 years (or 8 half-lives), the amount left is starting to become very small, less than 1%.

For the record, a beta-particle is a specific type of nuclear decay. Image 1 shows carbon-14 production by high energy neutrons hitting nitrogen-14 atoms, while in Image 2, carbon-14 naturally decomposes through beta-particle production.