Argon40

Argon-40, often written Argon40, is the most abundant stable isotope of the noble gas argon (Ar). It has 18 protons and 22 neutrons, giving a mass number of 40. Argon-40 is one of the three stable argon isotopes, the others being Argon-36 and Argon-38. In Earth's atmosphere, Argon-40 accounts for approximately 99.6% of natural argon.

Argon-40 is formed primarily by radiogenic processes in the crust and mantle, notably from the decay of radioactive potassium-40. Over geological timescales, argon produced in rocks is released into the atmosphere, where Argon-40 becomes the dominant isotope. It is chemically inert, like other isotopes of argon, and does not participate in chemical reactions under ordinary conditions.

In natural argon, Argon-40 is vastly more abundant than the other isotopes, which makes it the reference isotope for a range of measurements. In scientific practice, Argon-40 is central to radiometric dating methods such as argon-argon dating, where the ratio of radiogenic Argon-40 to proxy isotopes is used to determine ages of minerals and rocks after sample irradiation to produce the necessary isotopic proxies.

Industrial and research uses reflect the inert nature of argon generally: Argon-40 is the dominant isotope encountered in gas mixtures, serves as a calibration standard for mass spectrometry, and underpins detectors and processes that rely on stable, nonreactive noble gases.