C-14, or carbon-14, is a radioactive isotope of carbon with a nucleus containing six protons and eight neutrons. It is formed in the atmosphere when cosmic rays interact with nitrogen-14. C-14 is present in trace amounts in the environment and is absorbed by living organisms through processes like respiration and photosynthesis. When an organism dies, it stops taking in C-14, and the isotope begins to decay at a known rate, called its half-life, which is about 5,730 years. This property makes C-14 useful for dating ancient organic materials, a method known as radiocarbon dating, which helps scientists study historical artifacts and fossils.