Atoms of carbon-12 have 6 neutrons, while atoms of carbon-14 contain 8 neutrons.
neutral atom would have the same number of protons and electrons, so a neutral atom of carbon-12 or carbon-14 would have 6 electrons.
The other two isotopes in comparison are more common than carbon-14 in the atmosphere but increase with the burning of fossil fuels making them less reliable for study (2); carbon-14 also increases, but its relative rarity means its increase is negligible. After this point, other Absolute Dating methods may be used.
Today, the radiocarbon-14 dating method is used extensively in environmental sciences and in human sciences such as archaeology and anthropology.
So, we have a “clock” which starts ticking the moment something dies.