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Carbon-12, often abbreviated as \({}^{12}C\), is the most common isotope of carbon. It has 6 protons and 6 neutrons, which makes it stable and non-radioactive. This stability means that carbon-12 does not decay over time. Because of its stability, carbon-12 forms the primary building block of organic molecules in living organisms. Nearly all carbon found in living tissue is carbon-12.

Carbon-14, also known as \({}^{14}C\), is a radioactive isotope of carbon. It has 6 protons and 8 neutrons. Unlike carbon-12, carbon-14 is unstable, meaning it decays over time at a known rate. This decay process is what makes carbon-14 useful for radiocarbon dating. As living organisms absorb carbon dioxide from the atmosphere, they take in carbon-14 alongside carbon-12. However, the amount of carbon-14 is much smaller compared to carbon-12. Despite this, the ratio of carbon-12 to carbon-14 remains relatively constant in living organisms due to the continuous exchange of carbon with the environment.

The isotope ratio refers to the relative proportion of different isotopes of an element in a sample. For carbon isotopes in living tissues, the significant ratio is that of carbon-12 to carbon-14. This ratio is important for various scientific applications. For example, in radiocarbon dating, scientists use the known constant ratio of carbon-12 to carbon-14 to determine the age of archaeological and geological samples. The reason this ratio remains constant in living organisms is due to the constant exchange of carbon with their environment. When an organism dies, it stops absorbing carbon, and the carbon-14 begins to decay without being replenished. By measuring the remaining amount of carbon-14 in a sample and comparing it to the constant ratio found in living organisms, scientists can estimate how long it has been since the organism's death.