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Argon (18 Ar) has 26 known isotopes, from 29 Ar to 54 Ar, of which three are stable (36 Ar, 38 Ar, and 40 Ar). On the Earth, 40 Ar makes up 99.6% of natural argon. The longest-lived radioactive isotopes are 39 Ar with a half-life of 268 years, 42 Ar with a half-life of 32.9 years, and 37 Ar with a half-life of 35.04 days.
Ar is the reason the standard atomic weight of terrestrial argon is greater than that of the next element, potassium, a fact that was puzzling when argon was discovered. Mendeleev positioned the elements on his periodic table in order of atomic weight, but the inertness of argon suggested a placement before the reactive alkali metal .
Argon–argon (or 40 Ar/ 39 Ar) dating is a radiometric dating method invented to supersede potassium–argon (K/Ar) dating in accuracy. The older method required splitting samples into two for separate potassium and argon measurements, while the newer method requires only one rock fragment or mineral grain and uses a single measurement of argon isotopes.
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Argon forms a clathrate with hydroquinone (HOC 6 H 4 OH) 3 •Ar. [172] When crystallised from benzene under a pressure of 20 atmospheres of argon, a well defined structure containing argon results. [173] An argon-phenol clathrate 4C 6 H 5 OH•Ar is also known.
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Ohio County is a county located in the Northern Panhandle of the U.S. state of West Virginia, and forms part of the Wheeling metropolitan area. As of the 2020 census , the population was 42,425. [ 1 ]