Argon Element Facts

A faint line from argon shows in the spectrum of the doomed star Eta Carinae. William Ramsay discovered argon when he first saw its spectrum and realized it matched no other known gas.
Image: NASA, ESA, and the Hubble SM4 ERO Team

Classification:	Argon is a noble gas and a nonmetal
Atomic weight:	39.948 g/mol
Density @ 20 oC:	0.001784 g/cm3
Atomic volume:	22.4 cm3/mol

Nasa: Glowing argon created by the Cassiopeia A supernova, 10 000 light-years away in our own galaxy.

The percentage of each gas in earth's dry atmosphere. In practice water vapor is also present. Image: Mysid

Cavendish sparked air to react N₂ with O₂. Here's the same thing happening on a larger scale. Nitrogen oxides form, which fall to the ground as dilute nitric and nitrous acids in rainwater. The nitrate and nitrite ions in these acids are plant food. (This is part of the nitrogen cycle.) Image: Dani Toth

Discovery of Argon

Argon was the first noble gas to be discovered.

The first hint of its existence came from Sir Henry Cavendish as far back as 1785. Cavendish was unhappy that so little was known about air. He was particularly unhappy about the lack of information about the fraction of air (the majority) which was not oxygen. ⁽¹⁾

He knew the nitrogen in air could be reacted with oxygen to form, ultimately, nitrous acid. He aimed to find out if ALL of the air that was not oxygen or carbon dioxide could be converted to nitrous acid. If it could, he would know that air was entirely oxygen, carbon dioxide and nitrogen.

Cavendish used an electric spark in air to react the oxygen and nitrogen to form nitrogen oxides. He then added additional oxygen until all the nitrogen had reacted.

Nitrogen oxides are acidic. Cavendish used aqueous sodium hydroxide to remove them from the apparatus. [This would also, of course, have removed any carbon dioxide that was present.] He removed the remaining oxygen using potassium polysulfides.

A small bubble of gas remained [mostly argon]. Cavendish wrote that this bubble "was not more than one hundred and twentieth of the bulk of the phlostigated air [nitrogen]." ⁽¹⁾ So, Cavendish is saying that air is at least 99.3 percent nitrogen/oxygen/carbon dioxide with a maximum 0.7 percent of something else. We now know that the 'something else,' argon, is very unreactive; this enabled Cavendish to find it, but it also prevented him finding out more about it. (The giant advances in spectroscopy made by Gustav Kirchhoff and Robert Bunsen lay 85 years in the future.)

In hindsight, we can say Cavendish slightly underestimated the part of air that isn't oxygen, nitrogen, or carbon dioxide. Despite this, he was ahead of his time. After his experiment, more than 100 years passed until scientists again began to think that something about air didn't quite add up.

In 1892 John William Strutt (better known as Lord Rayleigh) announced that no matter how it was prepared, oxygen was always 15.882 times denser than hydrogen. This very precise work had taken ten years to complete.

Continuing to work with great attention to detail, he found that the 'nitrogen' in air was always denser by about 0.5 percent than nitrogen sourced from nitrogen compounds. ^{(2), (3)} How could this be explained? In 1893 he wrote to Nature, announcing the problem to the world. Any scientist who responded to that challenge actually had the chance of discovering a new element. None did!

In April 1894 Rayleigh wrote an academic paper about the nitrogen problem. Funnily enough, Rayleigh viewed pure nitrogen, containing no argon, as 'abnormally light nitrogen.' He stored it for eight months and retested it to see whether its density would increase. ⁽⁴⁾

Rayleigh's paper awakened the serious interest of William Ramsay, who had already been aware of the problem.

Rayleigh and Ramsay carried out further experiments, keeping in touch with one another about their progress.

In August 1894 Ramsay took air and removed its components - oxygen, carbon dioxide and nitrogen. He removed the nitrogen by reacting it with magnesium. After removing all the known gases from air, he found gas remaining that occupied one-eightieth of the original volume. Its spectrum matched no known gas.

Rayleigh and Ramsay wrote a joint paper in 1895 notifying the world of their discovery. The new gas wouldn't react with anything, so they named it argon, from the Greek 'argos,' meaning inactive or lazy. ⁽⁵⁾

In his Nobel Prize winning address, Rayleigh said, "Argon must not be deemed rare. A large hall may easily contain a greater weight of it than a man can carry." ⁽⁶⁾

William Ramsay discovered or codiscovered most of the other noble gases: helium, neon, krypton and xenon.

He was responsible for adding an entire new group to the periodic table. Radon was the only noble gas he didn't discover.

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