The Periodic Table

Argon

Symbol: Ar
Electron configuration: [Ne] 3s23p6
Atomic mass: 39.948 u
Boiling point: -185.8 °C
Atomic number: 18
Did you know: Argon is used as a motionless gas shield for arc welding and cutting.

Argon is a chemical element with symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in the Earth's atmosphere, at 0.934%. It is more than twice as abundant as water vapour, 23 times as abundant as carbon dioxide, and more than 500 times as abundant as neon. Argon is the most abundant noble gas in Earth's crust, comprising 0.00015% of the crust.

Nearly all of the argon in the Earth's atmosphere is radiogenic argon-40, derived from the decay of potassium-40 in the Earth's crust. In the universe, argon-36 is by far the most common argon isotope, as it is the most easily produced by stellar nucleosynthesis in supernovas.

The name "argon" is derived from the Greek word ἀργόν, neuter singular form of ἀργός meaning "lazy" or "inactive", as a reference to the fact that the element undergoes almost no chemical reactions. The complete octet (eight electrons) in the outer atomic shell makes argon stable and resistant to bonding with other elements.

Argon is a colourless, odourless gas that is totally inert to other substances.

Argon is often used when an inert atmosphere is needed. It is used in this way for the production of titanium and other reactive elements. It is also used by welders to protect the weld area and in incandescent light bulbs to stop oxygen from corroding the filament.

Argon is used in fluorescent tubes and low-energy light bulbs. A low-energy light bulb often contains argon gas and mercury. When it is switched on an electric discharge passes through the gas, generating UV light. The coating on the inside surface of the bulb is activated by the UV light and it glows brightly.

Double-glazed windows use argon to fill the space between the panes. The tyres of luxury cars can contain argon to protect the rubber and reduce road noise.

Argon has no known biological role.

Although argon is abundant in the Earth's atmosphere, it evaded discovery until 1894 when Lord Rayleigh and William Ramsay first separated it from liquid air. In fact, the gas had been isolated in 1785 by Henry Cavendish who had noted that about 1% of air would not react even under the most extreme conditions. That 1% was argon.

Argon was discovered as a result of trying to explain why the density of nitrogen extracted from air differed from that obtained by the decomposition of ammonia.

Ramsay removed all the nitrogen from the gas he had extracted from air, and did this by reacting it with hot magnesium, forming the solid magnesium nitride. He was then left with a gas that would not react and when he examined its spectrum he saw new groups of red and green lines, confirming that it was a new element.