Argon (Ar)

Argon (symbol Ar) is the chemical element with atomic number 18: a noble gas, with an atomic mass of 39.95 u. Discovered in 1894 (Lord Rayleigh and William Ramsay).

Argon is a noble gas that reacts with almost nothing, hence its Greek name, “the lazy one.” It makes up nearly 1% of the atmosphere, more than twenty times as much as CO₂. This inertness makes it the ideal gas for protecting things that must not react: welds, the inside of light bulbs or an opened bottle of wine.

Key facts

Atomic number: 18
Symbol: Ar
French name: Argon
Atomic mass: 39.95 u
Molar mass: 39.95 g/mol
Family: Noble gases
Position: period 3, group 18, p-block
State at 0 °C: Gas
Melting point: -189.3 °C
Boiling point: -185.8 °C
Density: 0.00178 g/cm³
First ionization energy: 1,521 kJ/mol
Atomic radius: 71 pm
Electron configuration: [Ne] 3s² 3p⁶
Abundance in Earth's crust: 3.5 ppm (43rd most abundant element)
Indicative price: 0.93 $/kg (order of magnitude)
Radioactive: no
Discovery: 1894, Lord Rayleigh and William Ramsay

Origin of the name

From the Greek argos, inactive, lazy.

What does argon look like?

A colorless, odorless gas. In a discharge tube it gives off a violet to lavender-blue glow.

Where is argon found?

0.93% of the atmosphere, the third gas in air after nitrogen and oxygen. Almost all of Earth's argon (argon-40) comes from the radioactive decay of potassium-40 in rocks.

Natural sources

Air (0.93%)

How is argon produced?

Separated by distilling liquefied air, as a by-product of oxygen and nitrogen production. It is the cheapest noble gas, because it is the most abundant.

What is argon used for?

Shielding gas for welding (TIG, MIG).
Filling incandescent bulbs and fluorescent tubes.
Insulation in double glazing.
Refining stainless steel.
Preserving wine and food.
Potassium-argon geological dating.

In everyday life

The double glazing in your windows
Old-fashioned filament light bulbs
Wine-preserver spray cans for opened bottles

Role in living things

No biological role. We breathe it in and out with every breath without it interacting with the body.

Hazards of argon

Non-toxic, but heavier than air: it can pool in pits or tanks and cause silent suffocation. Liquid argon (−186 °C) causes cold burns.

Cosmic origin

Supernovae: Forged in the cores of massive stars, then scattered when they explode.
White dwarfs: Produced by the thermonuclear explosion of a white dwarf (type Ia supernova).

History of argon

In 1892, Lord Rayleigh noticed that nitrogen taken from air was slightly heavier than nitrogen produced chemically. With William Ramsay, he deduced in 1894 the presence of an unknown gas, argon. The discovery earned each of them a Nobel Prize in 1904 and opened the way to the whole family of noble gases.

Did you know?

A 50 m³ room contains about 1 kg of argon.
Argon is more abundant in air than carbon dioxide, about 22 times more.
It was the first noble gas discovered, at a time when Mendeleev's table had no column for it.

Isotopes of argon

Three stable isotopes, including argon-40, which makes up 99.6% of the argon in air. Nearly all of it comes from the decay of potassium-40 in rocks.

argon-36: 0.334% of natural argon, stable
argon-37: radioactive, half-life 35 days. Created by solar neutrinos striking chlorine-37, it revealed the existence of neutrinos from the Sun.
argon-38: 0.063% of natural argon, stable
argon-39: radioactive, half-life 268 years. Dates groundwater and ocean water over a few centuries.
argon-40: 99.604% of natural argon, stable. Building up in rocks, it makes potassium-argon dating possible.
argon-41: radioactive, half-life 1.83 h. Formed in the air around reactors, it is used to measure gas flow rates in industry.

Related elements

Same family (noble gases): Helium (He), Neon (Ne), Krypton (Kr), Xenon (Xe), Radon (Rn).

Same column (group 18): Helium (He), Neon (Ne), Krypton (Kr), Xenon (Xe), Radon (Rn), Oganesson (Og).