Argon

Argon is the chemical element with atomic number 18 and the symbol Ar. It is a colorless, odorless noble gas in group 18 of the periodic table, and it makes up almost 1 percent of Earth’s atmosphere. Argon is chemically inert, so it is used wherever a non-reactive atmosphere is needed.

History of argon

Argon is plentiful in the air, yet it escaped discovery until 1894. In 1785 the English scientist Henry Cavendish had noticed that about 1 percent of air would not react even under extreme conditions. That leftover fraction was argon, but nobody followed up on his observation for more than a century.

The discovery came from a puzzle about density. Lord Rayleigh found that nitrogen taken from air was slightly denser than nitrogen made by breaking down ammonia. The Scottish chemist William Ramsay removed all the nitrogen from a sample of air by reacting it with hot magnesium, which forms solid magnesium nitride. A gas remained that would not react with anything, and its spectrum showed new red and green lines.

Rayleigh and Ramsay announced the new element in 1894. The name comes from the Greek argos, meaning idle, in reference to its lack of reactivity. Both men received Nobel Prizes in 1904, Rayleigh in physics and Ramsay in chemistry.

Properties of argon

Argon has a standard atomic weight of about 39.95 and an electron configuration of [Ne] 3s2 3p6. Its outer shell is completely full, which explains the lack of reactivity. PubChem lists a first ionization energy of 15.760 electronvolts, meaning it takes a great deal of energy to remove an electron.

Argon is a gas under ordinary conditions. It condenses to a liquid at about minus 185.8 degrees Celsius and freezes at about minus 189.3 degrees Celsius. At standard conditions it is slightly denser than air, with a density of about 0.0018 grams per cubic centimeter. Argon forms very few compounds, and those that exist are stable only at extremely low temperatures.

One oddity concerns the atomic weight. Argon has a higher atomic weight than potassium, which sits after it in the periodic table with a higher atomic number. This happens because the dominant isotope of argon is heavier than the dominant isotope of potassium. It is an early example of why elements are ordered by atomic number, not by weight.

Occurrence and production of argon

Argon makes up 0.94 percent of the atmosphere by volume, which makes it the third most abundant gas in the air after nitrogen and oxygen. Its share has risen gradually since Earth formed, because the radioactive isotope potassium-40 decays into argon-40 inside rocks, and some of that gas escapes into the air. The thin atmosphere of Mars also contains argon, at close to 2 percent.

Argon is produced commercially from the air itself. Liquid air is separated by distillation into its components, and argon is collected as a byproduct of the large plants that make oxygen and nitrogen. Because it is extracted from air, supply is not tied to a mineral deposit and the gas is relatively inexpensive.

Uses of argon

Argon is used mainly where an inert atmosphere is needed. Metal producers use it to make titanium and other reactive metals that would otherwise combine with air. Welders use it to shield the weld area, because molten metal in an argon atmosphere does not oxidize. Gas tungsten arc welding, also known as TIG welding, relies on this shielding gas.

Argon fills incandescent light bulbs, where it stops oxygen from corroding the hot filament. It is also used in fluorescent tubes and low-energy bulbs, which often contain argon with a little mercury. When the lamp is switched on, an electric discharge passes through the gas and generates ultraviolet light, which makes the coating on the inside of the glass glow brightly.

Double-glazed windows are often filled with argon in the gap between the panes. Argon conducts heat less well than air, which improves insulation, and the gas also keeps moisture out of the sealed space. A further use is in growing silicon crystals for electronics, where a clean, unreactive atmosphere is essential, and some museums use argon to protect fragile documents from oxidation. The same inertness makes it useful in glass production and in analytical instruments.

Argon ion lasers produce intense blue and green light and have been used in scientific research, in entertainment light shows and in eye surgery. Argon is also used in some fire suppression systems for rooms that hold sensitive equipment, because it lowers the oxygen level without leaving any residue.

Isotopes of argon

Argon has three stable isotopes: argon-36, argon-38 and argon-40. Argon-40 dominates, at about 99.6 percent of the argon in the atmosphere, while argon-36 makes up about 0.33 percent and argon-38 about 0.06 percent. Most atmospheric argon-40 came from the decay of potassium-40.

This decay is the basis of potassium-argon dating. When rock is molten, argon escapes, but once it solidifies the argon produced by decaying potassium is trapped. Measuring the ratio of the two reveals the age of volcanic rock. Potassium-40 has a half-life of about 1.25 billion years, so the method works for samples ranging from thousands to billions of years old. A related method uses argon-39. See the entries on isotope and half-life for more.

Health and environmental effects of argon

Argon has no known biological role and is not toxic. As with other inert gases, however, large releases in enclosed spaces can displace oxygen, and workplace safety agencies treat this as an asphyxiation hazard. The information here is general background rather than safety advice.

Because argon is a natural component of air and does not react, it has no significant environmental impact. It is not a greenhouse gas and does not harm the ozone layer.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed November 4, 2026Researched and drafted with AI assistanceReport an issue