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Argon (Greek ἀργόν, neuter singular form of ἀργός meaning "lazy" or "inactive") is named in reference to its chemical inactivity. This chemical property of this first noble gas to be discovered impressed the namers. [16] [17] An unreactive gas was suspected to be a component of air by Henry Cavendish in 1785. [18]
In 2003, using argon–argon dating, the Upper Herto Member was dated to 160 to 154 thousand years ago. [3] The Herto Man was a major fossil find as, at the time, there was a significant gap in the human fossil record between 300 and 100 thousand years ago, obfuscating the evolution of "Homo (sapiens?) rhodesiensis" into H. s. sapiens. [1]
In mammals, including humans, it is an important cellular signalling molecule involved in many physiological and pathological processes. [68] It is formed by catalytic oxidation of ammonia. It is a colourless paramagnetic gas that, being thermodynamically unstable, decomposes to nitrogen and oxygen gas at 1100–1200 °C.
Perey discovered it as a decay product of 227 Ac. [178] Francium was the last element to be discovered in nature, rather than synthesized in the lab, although four of the "synthetic" elements that were discovered later (plutonium, neptunium, astatine, and promethium) were eventually found in trace amounts in nature as well. [179]
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.
A problem now arose – Mendeleev had not left any gaps which were suitable for either of these two new elements, which led Ramsay to conclude an entire group was missing from the periodic table – only two of whose members were now known to exist, helium and argon. Ramsey successfully discovered all the other stable elements in the group ...
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]
A 100,000-year-old ochre-processing workshop was found at Blombos Cave in South Africa. It indicates that early humans had an elementary knowledge of mineral processing. Paintings drawn by early humans consisting of early humans mixing animal blood with other liquids found on cave walls also indicate a small knowledge of chemistry. [3] [4]