Argon-40 is a stable isotope of the element argon, which is a noble gas found in the Earth's atmosphere. It has an atomic mass of 40 and makes up about 0.934% of natural argon. Argon-40 is produced through the radioactive decay of potassium-40 and is commonly used in various scientific applications, including dating geological samples. In addition to its use in geology, argon-40 is also important in noble gas chromatography and as a shielding gas in welding. Its inert nature makes it ideal for protecting reactive materials during chemical processes. Overall, argon-40 plays a significant role in both research and industrial applications.