Carbon-14 (\(\text{C-14}\)) is a naturally occurring radioactive isotope of carbon. It contains the standard six protons of carbon, but with eight neutrons instead of the six found in stable Carbon-12 (\(\text{C-12}\)). This excess of neutrons makes the nucleus unstable, causing it to naturally undergo radioactive decay to achieve a more stable configuration. This decay process reveals the stable element Carbon-14 transforms into and provides a powerful tool for measuring time in the natural world.
The Decay Transformation
The unstable Carbon-14 atom transforms into a stable element through a process called beta decay. This transformation occurs when one of the eight neutrons within the carbon nucleus spontaneously converts into a proton. This nuclear change increases the atomic number by one, fundamentally altering the identity of the atom.
The newly formed nucleus now contains seven protons and seven neutrons, which is the makeup of stable Nitrogen-14 (\(\text{N-14}\)). During this process, the nucleus ejects an electron, known as a beta particle, and an electron antineutrino. Because Nitrogen-14 is a stable isotope, it will not undergo any further radioactive transformation.
The Atmospheric Creation and Biological Uptake
Carbon-14 exists in the environment because it is continuously created in the Earth’s upper atmosphere. This creation begins when high-energy cosmic rays collide with gas molecules in the stratosphere. These collisions generate a stream of thermal neutrons that interact with Nitrogen-14 (\(\text{N-14}\)) atoms, the most abundant gas in the atmosphere.
A nitrogen-14 atom absorbs a neutron and simultaneously ejects a proton, which converts it into a Carbon-14 atom. The newly formed C-14 atoms quickly react with atmospheric oxygen, forming radioactive carbon dioxide (\(\text{CO}_2\)). This radioactive \(\text{CO}_2\) then mixes thoroughly with the far more abundant \(\text{CO}_2\) containing the stable isotopes, Carbon-12 and Carbon-13.
Living organisms constantly absorb carbon from the atmosphere and food sources. Plants incorporate both stable Carbon-12 and radioactive Carbon-14 through photosynthesis. Animals then consume these plants or other animals, incorporating the same ratio of C-14 to C-12 into their tissues.
This constant exchange ensures that all living things maintain a constant ratio of Carbon-14 to stable carbon isotopes. This equilibrium means the C-14 that decays within a living organism is immediately replaced by new C-14 from the environment. The moment an organism dies, however, this replenishment stops, and the radioactive clock begins ticking.
Measuring Time with Ratios
The predictable rate at which Carbon-14 decays is the basis for the technique known as radiocarbon dating. Once an organism dies, the C-14 already present in its tissues begins to decay into stable Nitrogen-14 without being replaced. The amount of stable Carbon-12 in the sample remains constant, but the amount of radioactive C-14 decreases over time.
Scientists measure this decay using the concept of a half-life, which is the time required for half of the radioactive atoms in a sample to transform. Carbon-14 has a half-life of 5,730 years, meaning that after this period, only half of the original C-14 remains in the sample. After another 5,730 years, only one-quarter of the initial amount is left.
The dating method involves comparing the remaining ratio of Carbon-14 to Carbon-12 in a dead sample to the ratio that existed when the organism was alive. By measuring the fraction of C-14 that has disappeared, researchers calculate how many half-life periods have passed since the organism died. This allows for the estimation of the time elapsed since the organism ceased exchanging carbon with its environment.
Because the amount of remaining Carbon-14 becomes small over time, the technique has practical limitations. Radiocarbon dating is effective for determining the age of organic materials up to approximately 50,000 to 60,000 years old. Beyond this range, the remaining C-14 is often too minute to provide a reliable measurement.