Carbon has three isotopes: carbon-12, carbon-13 and carbon-14
There is always a tiny fixed proportion of C-14 in all living matter. Once the living matter dies carbon stops being taken in from food and CO2 is no longer given out by respiration if decay does not occur so this proportion decreases over time.The half-life of carbon-14 is 5730 years so the amount of carbon-14 halves every 5730 years and the proportion changes.
| carbon-12 | carbon-13 | carbon-14 | |
| symbol | 12 C 6 | 13 C 6 | 14 C 6 |
| protons | 6 | 6 | 6 |
| neutrons | 6 | 7 | 8 |
| abundance | 98.89% | 1.11% | trace |
There is always a tiny fixed proportion of C-14 in all living matter. Once the living matter dies carbon stops being taken in from food and CO2 is no longer given out by respiration if decay does not occur so this proportion decreases over time.The half-life of carbon-14 is 5730 years so the amount of carbon-14 halves every 5730 years and the proportion changes.