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Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas . [ 10 ] Argon is the third most abundant gas in Earth's atmosphere , at 0.934% (9340 ppmv ).
Almost all argon in the Earth's atmosphere is the product of 40 K decay, since 99.6% of Earth's atmospheric argon is 40 Ar, whereas in the Sun and presumably in primordial star-forming clouds, argon consists of < 15% 38 Ar and mostly (85%) 36 Ar. Similarly, the ratio of the isotopes 36 Ar: 38 Ar: 40 Ar in the atmospheres of the outer planets is ...
The argon found in Earth's atmosphere is 99.6% 40 Ar; whereas the argon in the Sun – and presumably in the primordial material that condensed into the planets – is mostly 36 Ar, with less than 15% of 38 Ar. It follows that most of Earth's argon derives from potassium-40 that decayed into argon-40, which eventually escaped to the atmosphere.
The abundance of argon, on the other hand, is increased as a result of the beta decay of potassium-40, also found in the Earth's crust, to form argon-40, which is the most abundant isotope of argon on Earth despite being relatively rare in the Solar System. This process is the basis for the potassium-argon dating method. [72]
Argon (symbol Ar) is the third element in group 18, the noble gases. Argon is the third most common gas in the Earth's atmosphere , at 0.93%, making it more common than carbon dioxide . Nearly all of this argon is radiogenic argon-40 derived from the decay of potassium-40 in the Earth's crust.
The decay of 40 K to 40 Ar is used in potassium-argon dating of rocks. Minerals are dated by measurement of the concentration of potassium and the amount of radiogenic 40 Ar that has accumulated. Typically, the method assumes that the rocks contained no argon at the time of formation and all subsequent radiogenic argon (i.e., 40 Ar) was retained.
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