C13, or Carbon-13, is a stable isotope of carbon that contains six protons and seven neutrons in its nucleus. It makes up about 1.1% of all carbon found in nature. Carbon-13 is commonly used in various scientific fields, including chemistry and biology, for applications such as nuclear magnetic resonance (NMR) spectroscopy, which helps identify molecular structures. This isotope is also important in studies of metabolic processes and environmental science. By tracking the behavior of Carbon-13 in different compounds, researchers can gain insights into processes like photosynthesis and carbon cycling in ecosystems.