K-40, or potassium-40, is a naturally occurring isotope of potassium. It is radioactive and makes up about 0.012% of all potassium found in nature. K-40 has a long half-life of approximately 1.25 billion years, which means it decays very slowly over time. This isotope is significant in geology and archaeology for dating rocks and fossils through a process called potassium-argon dating. K-40 emits beta particles and gamma radiation as it decays into stable isotopes, such as argon-40. Because potassium is an essential nutrient for living organisms, K-40 is present in various foods, including fruits and vegetables. Its natural abundance and long half-life make it an important element in studies related to radiometric dating and understanding Earth's geological history.