Argon is a colorless, odorless, inert noble gas that makes up about 0.93% of Earth’s atmosphere. It is chemically nonreactive under most conditions and is used widely in shielding atmospheres for welding, in incandescent and fluorescent lighting, and in scientific and industrial processes.
Argon has a complete valence shell with configuration \([Ne]3s^2\,3p^6\). This closed octet gives it a very high ionization energy and no tendency to gain or lose electrons, so Ar rarely forms stable compounds under ordinary conditions.
Argon is isolated by fractional distillation of liquefied air. After removing CO2 and H2O, air is liquefied and separated in distillation columns; Ar is drawn from the oxygen–argon fraction and further purified.
Because Ar is inert and denser than air, it blankets the hot metal, excluding oxygen, nitrogen, and moisture that would otherwise cause oxidation, nitrides, or porosity. Its low thermal conductivity also stabilizes the arc in processes like GTAW/TIG and GMAW/MIG.
In incandescent bulbs, Ar (often with a little N2) suppresses tungsten filament evaporation, extending life. In discharge tubes, excited Ar emits characteristic lines (bluish–violet). Photon energy relates to wavelength by \(E = h\nu = \dfrac{hc}{\lambda}\), so different lines produce different colors.
Stable isotopes are \(^{36}\!\mathrm{Ar}\), \(^{38}\!\mathrm{Ar}\), and \(^{40}\!\mathrm{Ar}\). Most atmospheric argon is \(^{40}\!\mathrm{Ar}\), generated by radioactive decay of \(^{40}\!\mathrm{K}\) in rocks:
\(\mathrm{^{40}K \xrightarrow{EC/\beta^+} \,^{40}Ar}\)
This decay is the basis of K–Ar and Ar–Ar geochronology.
Argon is extremely unreactive, but under specialized conditions (matrix isolation at cryogenic temperatures or with strong electrophiles) transient species such as argon clathrates or weakly bound adducts can be observed. These are not stable, isolable compounds under normal conditions.
Liquid argon boils near 87 K at 1 atm. It is used as a cryogen, in particle detectors (LAr scintillation/ionization for neutrino and dark-matter experiments), and as an inert coolant when ultra-low reactivity is required.
Argon is non-toxic, but it can displace oxygen in confined spaces and cause asphyxiation without warning. Ensure good ventilation, use oxygen monitors in enclosed areas, and secure high-pressure cylinders upright with appropriate regulators.
Although N2 is relatively inert, it can react at high temperatures (e.g., with certain metals to form nitrides) and can quench plasmas differently. Argon’s greater inertness and higher density make it better for shielding molten metals and for plasma and sputtering processes in materials science.
An argon-ion laser is a gas laser where Ar is ionized in a high-current discharge; stimulated emission occurs on specific lines (famously near 488 nm and 514.5 nm), producing intense blue–green light used historically in microscopy, holography, and printing.