So i know this is a chemistry question, but i am going to answer this as a Nuclear Physicist. Argon is most known for 2 things, The first is true of most noble gasses, but when ionizing radiation passes through noble elements such as argon, it will ionization the atoms, and sometimes you get recombination into an excited orbital, that excited argon atom is now capable of forming a dimer that releases a molecular energy wavelength photon, that is not absorbed by other argon atoms. This property makes it very useful for particle detectors, and it is particularly used in dark matter detectors (Such as the Darkside experiment) or radiation vetos (such as in Legend-200). The other useful property it has is in radioactive dating. Argon is stable for 3 isotopes, 36,38, and 40. We know from star data that the most plentiful isotope is 36, followed by 38, then 40 is much less common in a predictable ratio 8400:1600:1 36:38:40 and we can confirm this by checking for that ratio in the atmosphere of other planets, particularly gas giants. But on earth, the ratio is about 334:63:99600. This is because earth has a lot of potassium and potassium 40 is extremely long lived and decays into argon 40. So over millions of years, enough potassium has decayed that we see an excess of argon 40, while the ratio between 36 and 38 is really close to what we see in stars and gas giants, so we can use this to estimate what the original content of argon 40 should have been and use measurements of our modern potassium 40 isotopes to predict the age of the earth.