Final answer:
The elements measured to determine the age of an organism's remains are carbon-14, nitrogen, and carbon-12. Carbon-14 decays into nitrogen after the organism's death, while carbon-12 remains stable, allowing geologists to calculate the time since death by comparing their ratios.
Step-by-step explanation:
The three elements that geologists measure to determine the age of an organism's remains are carbon-14, nitrogen, and carbon-12. When an organism is alive, it constantly absorbs carbon, including the radioactive isotope carbon-14, as well as the stable isotopes carbon-12 and carbon-13. After the organism dies, it ceases to absorb carbon, and the carbon-14 it contains begins to decay into nitrogen at a known rate, called the half-life. Because carbon-12 is stable, its amount remains constant in the organism's remains. This allows geologists to determine the time elapsed since the death of the organism by comparing the ratio of carbon-14 to carbon-12.
Radiocarbon dating relies on the fact that while an organism is alive, the ratio of carbon-14 to carbon-12 in its body is the same as the ratio in the atmosphere. After death, the carbon-14 decays, changing into nitrogen, while the amount of carbon-12 remains constant. The changed ratio of carbon-14 to carbon-12, therefore, indicates the amount of time that has passed since the organism's death. Other isotopes like potassium-40 can be used to date older fossils, but for recent remains, the carbon-14 method is the standard.