Argon

Argon is the third most abundant gas in the air we breathe, yet most people have never heard of it. It sits in every light bulb, every double-glazed window and every welding workshop in the world, doing its one perfect trick: absolutely nothing. Argon is chemistry's great refuser, it reacts with nothing.

  • Atomic Number1818 protons, 18 electrons
  • Atomic Mass39.9 uAbout 40× heavier than hydrogen
  • State at Room TempGascolourless, odourless gas
  • Density0.0017837 g/cm³About 1.4× heavier than air
  • Melting / Boiling-189.3°C / -185.8°CLiquefies at −186°C
  • Discovered1894Rayleigh & Ramsay, 1894

How does argon compare to the other noble gases in mass?

Argon is the most abundant noble gas in the atmosphere and sits in the middle of Group 18.

Atomic Mass Comparison
Neon20 u
Argon40 u
Krypton84 u
Iron56 u
Xenon131 u

Argon (40 u) is exactly ten times heavier than helium (4 u) and double the mass of neon (20 u). Krypton (84 u) is just over twice as heavy as argon. Despite these differences, all noble gases are essentially inert under normal conditions.

What is argon?

Argon is a noble gas in Group 18 of the periodic table, sitting between chlorine and potassium. It has 18 protons and 18 electrons, with its outer electron shell completely filled. That perfect electronic stability means argon has virtually no tendency to form bonds with any other element. It is the most abundant noble gas in the atmosphere, the cheapest noble gas to produce, and one of the most widely used industrial gases in the world.

Argon gets its name from the Greek word argos, meaning idle or lazy, a reference to its complete reluctance to react with anything. The name was proposed by William Ramsay and John William Strutt (Lord Rayleigh) when they discovered it in 1894. It was widely considered an appropriate name for a substance that stubbornly refused to do anything chemically interesting.

Fact Argon makes up nearly 1% of the air, far more abundant than carbon dioxide (0.04%). Yet because it is completely colourless, odourless and unreactive, its existence was completely unknown until 1894, even though millions of tonnes of it were floating in the air around chemists every day.

Where you find argon

In space and on Earth

Argon is produced when radioactive potassium-40 inside planets decays, which is why it accumulates in planetary atmospheres over geological time. Mars has an atmosphere that is approx. 1.9% argon. Earth's atmosphere is 0.93% argon: the third most abundant atmospheric gas after nitrogen and oxygen, and almost entirely the product of billions of years of radioactive potassium decay inside the planet's crust and mantle.

  • The atmosphere. Earth's air is approx. 0.93% argon by volume. A cubic metre of air contains approx. 9.3 litres of argon.
  • Industrial production. Argon is extracted from the atmosphere by liquefying air and separating the gases by their different boiling points. It is the cheapest noble gas to produce in bulk.

How we use argon

  • Light bulbs. Argon is pumped inside incandescent light bulbs to prevent the hot tungsten filament from burning away. Because argon does not react with tungsten even at extreme temperatures, the bulb lasts far longer than it would in air or a vacuum.
  • Double-glazed windows. Argon gas is sealed between the panes in double-glazed windows. It insulates better than air because its heavier molecules carry heat less efficiently, keeping buildings warmer and reducing energy bills.
  • Welding. Argon is used as a shielding gas in MIG and TIG welding. A stream of argon around the weld prevents oxygen and nitrogen from reacting with the hot metal, producing a cleaner, stronger weld.
  • Preserving precious documents. Argon is used in display cases for priceless historical documents such as the original Magna Carta. The inert atmosphere prevents any chemical deterioration.
Did you know? The argon in Earth's atmosphere was not here when the planet formed. Almost all of it has been produced over billions of years by the radioactive decay of potassium-40 trapped inside rocks. Geologists can use the ratio of argon-40 to potassium-40 in a rock to calculate exactly how old it is, a technique called potassium-argon dating used to date rocks billions of years old.

How it was discovered

Argon was discovered in 1894 by Lord Rayleigh and William Ramsay in London, after a puzzling discrepancy. Rayleigh had noticed that nitrogen extracted from air was always very slightly denser than nitrogen made from compounds. He and Ramsay suspected a heavier, unknown gas was mixed with the atmospheric nitrogen. By repeatedly passing air over hot magnesium (which absorbed the nitrogen) and over hot copper (which absorbed the oxygen), they concentrated a residue of gas that would not react with anything. This turned out to be argon: the first noble gas discovered.

Deeper dive: argon chemistry and the discovery of Group 18

Argon has one confirmed compound formed under any conditions: argon fluorohydride (HArF), created at extremely low temperatures in 2000. Under normal conditions, argon forms no compounds whatsoever. This makes it genuinely unique, even xenon and krypton form a handful of compounds, but argon chemistry is essentially non-existent.

The discovery of argon caused a genuine crisis in chemistry because the periodic table, as Mendeleev had arranged it, had no space for a completely new family of elements with these properties. The eventual recognition that a whole new group, Group 18, the noble gases, had been missed led to the discovery of helium, neon, krypton, xenon and radon in just four years (1895-1900), one of the most extraordinary periods in the history of chemistry.

Potassium-argon dating is one of the most important radiometric dating methods in geology. Potassium-40 has a half-life of 1.25 billion years. By measuring the ratio of argon-40 to potassium-40 in an igneous rock, geologists can determine when it solidified. This method has been used to date the oldest known rocks on Earth (approx. 4 billion years old) and to confirm the ages of fossil-bearing sediments.

Argon is the noble gas hiding in plain sight, making up nearly 1% of every breath. Moving to 19 protons brings us to potassium, an alkali metal essential for every heartbeat.