(Isotopes are atoms of the same element that have different numbers of neutrons in their nuclei. (scientificamerican.com)
This aging process can be seen as certain atoms «put on a little weight,» meaning they change into heavier isotopes — atoms with additional neutrons in their nuclei. (greenbankobservatory.org)
Fossil fuels tend to contain more carbon - 13, a carbon isotope with seven neutrons in its nucleus instead of the more common six. (sciencemag.org)