argon

argon
Name	argon
Number	18
Category	noble gas
Group	18
Appearance	colorless gas exhibiting a lilac/violet glow when placed in an electric field
Standard atomic weight	39.95
Phase at gas	gas
Melting point c	-189.34
Boiling point c	-185.848
Density gpcm3nbp	1.784
Triple point k	83.8058
Critical point k	150.687
Heat of fusion	1.18
Heat of vaporization	6.43
Molar heat capacity	20.85
Ionization energies	1520.6, 2665.8, 3931, 5771, 7238, 8781, 11995, 13842, 40760, 46186
Crystal structure	face-centered cubic
Thermal conductivity	0.01772
Magnetic ordering	diamagnetic
Cas number	7440-37-1
Isotopes	36Ar, 38Ar, 40Ar

argon. It is a chemical element with the symbol Ar and atomic number 18, positioned in group 18 of the periodic table, making it a noble gas. It is the third-most abundant gas in the Earth's atmosphere, constituting about 0.934% by volume, and is produced industrially by the fractional distillation of liquid air. This odorless, colorless, and inert gas is most commonly used in applications where an unreactive atmosphere is required, such as in incandescent lighting, welding, and the production of semiconductor materials.

Properties

Under standard conditions, argon is a monatomic gas with a complete valence shell, rendering it exceptionally chemically inert and placing it among the most stable elements known. Its physical properties include a boiling point of -185.848 °C and a melting point of -189.34 °C, with a density higher than that of air. The element exhibits a distinctive lilac or violet glow when electrically excited, a phenomenon utilized in gas-discharge lamps and plasma globes. Its thermal conductivity is relatively low, and it is diamagnetic, meaning it is repelled by magnetic fields. The most abundant isotope on Earth is ⁴⁰Ar, a product of the radioactive decay of ⁴⁰K in the planet's crust, a process central to potassium-argon dating methods used in geology and archaeology.

History

The existence of argon was first suspected by Henry Cavendish in 1785 during experiments with air, but it was not isolated until 1894 by Lord Rayleigh and Sir William Ramsay at University College London. Their discovery followed Rayleigh's meticulous measurements showing that nitrogen derived from the atmosphere was denser than nitrogen obtained from chemical compounds, leading them to postulate and then isolate a previously unknown, heavier component. Ramsay named the gas "argon," from the Greek word *argos* meaning "idle" or "inactive," reflecting its non-reactive nature. This groundbreaking work, which led to the identification of an entire new group on the periodic table, earned Rayleigh the Nobel Prize in Physics in 1904 and Ramsay the Nobel Prize in Chemistry the same year.

Occurrence and production

In the cosmos, argon is synthesized in stars through nucleosynthesis processes, including silicon burning and supernova explosions. On Earth, it accumulates in the atmosphere as ⁴⁰Ar, a radiogenic isotope from the decay of potassium-40 in minerals like biotite and hornblende. The primary industrial production method is the cryogenic fractional distillation of liquefied air, a process also used to isolate nitrogen, oxygen, and other rare gases like neon and krypton. Major air separation plants operated by companies like Linde, Air Liquide, and Air Products produce high-purity argon as a byproduct. It is typically stored and transported in grey-painted steel cylinders or as a cryogenic liquid in specialized Dewar flask vessels.

Compounds

True, stable chemical compounds of argon remain elusive under normal conditions due to its filled valence electron shell, which confers extreme inertness. However, theoretical predictions and experimental work under extreme conditions have hinted at possible bonding. In 2000, researchers at the University of Helsinki reported the first neutral compound containing argon, argon fluorohydride (HArF), which is stable only at very low temperatures. Other theoretical or weakly bound species include ions like ArH⁺, detected in interstellar space such as the Crab Nebula, and van der Waals molecules or clathrates, where argon atoms are trapped in cages of water or other host molecules without forming covalent bonds.

Applications

Its inertness makes argon invaluable for creating protective atmospheres. In metallurgy, it is used in argon oxygen decarburization for stainless steel production and as a shielding gas in TIG and MIG welding to protect molten metals from oxidation. The lighting industry employs it in incandescent bulbs to prevent filament degradation and in fluorescent tubes and neon signs for its distinct glow. In electronics, it provides an inert environment for growing silicon and germanium crystals for semiconductors. Other uses include preserving historical documents like the U.S. Constitution and the Magna Carta in display cases, as a carrier gas in gas chromatography, and as the propellant in some types of fire extinguisher systems.

Safety

Argon is classified as a simple asphyxiant, posing a primary hazard by displacing oxygen in confined spaces, which can lead to unconsciousness and death without warning. The Occupational Safety and Health Administration and other regulatory bodies set exposure limits and mandate monitoring in workplaces where argon is used in bulk. It is odorless and colorless, providing no sensory indication of a hazardous atmosphere, necessitating the use of oxygen monitors in at-risk areas like laboratories, welding shops, and facilities using cryogenic systems. While non-flammable and non-toxic, liquid argon presents cryogenic burn risks and can cause extreme pressure buildup if rapidly vaporized in a sealed container.

Category:Chemical elements Category:Noble gases Category:Industrial gases