From Chemistry Resource
Jump to: navigation, search

This is argon.

Symbol Ar
Atomic number 18
Atomic mass 390948 amu

Argon, from argon meaning "inactive", was discovered in 1894.


It was found by analyzing atmospheric gases after removing oxygen and nitrogen. When all traces of carbon dioxide, dust, and moisture was also removed, there was enough gas that remained such that it was suspected that an additional gas was present. It is the most common of the noble gases. Like the other noble gases it is colourless, odourless and tasteless. It makes up 0.934% of the air and is released continuously by the decay of radioactive potassium in rocks (by electron-capture of 40K). The amounts of argon formed by radioactive decay in minerals serves as a method to determine the age of the specimen. Shown in the image to the right is the glow discharge of argon.


Argon is used:

  • in filling incandescent light bulbs since the hot metallic filament tends to evaporate if the bulb is evacuated, and it reacts if other gas is used,
  • in argon-arc welding for joining aluminum or stainless steel. In the presence of air, these metals form thin layers of hard oxide which prevent the weld from taking hold. A stream of argon flowing around the tungsten electrode provides an inert atmosphere and protects the weld pool from oxygen as the joint is welded.

See also

Periodic table of the elements