Argon is the chemical element with symbol Ar and atomic number 18. It is a colourless, odourless and highly unreactive noble gas in group 18 of the periodic table. Argon makes up close to one percent of Earth’s atmosphere by volume, making it the most abundant noble gas in the air.
Its chemical inertness makes argon extremely useful whenever oxygen, nitrogen or moisture would interfere with a process. It is widely used in welding, metal production, lighting, laboratories, semiconductor manufacture and protective atmospheres.
Argon quick facts
| Property | Value |
| Symbol | Ar |
| Atomic number | 18 |
| Relative atomic mass | 39.95 |
| Group | 18 |
| Period | 3 |
| Block | p-block |
| Classification | Noble gas |
| State at 20°C | Gas |
| Appearance | Colourless, odourless |
| Density | 0.001633 g/cm³ |
| Melting point | −189.34°C |
| Boiling point | −185.848°C |
| Electron configuration | [Ne] 3s² 3p⁶ |
| CAS number | 7440-37-1 |
| Key natural isotope | ⁴⁰Ar |
Where is argon on the periodic table?
Argon is in group 18, period 3 and the p-block. Its electron configuration is [Ne] 3s² 3p⁶. This filled outer electron shell makes neutral argon atoms exceptionally stable and explains the element’s very low chemical reactivity.
See the complete CleverlySmart periodic table. Argon lies below neon (Ne) in the noble-gas group.
Physical properties of argon
Argon is a monatomic gas under ordinary conditions. It has no colour or smell and is denser than air. At very low temperatures it condenses to a colourless liquid and then freezes. Its boiling point is approximately −185.848°C, only a few degrees above its melting point of −189.34°C.
Argon does not burn and does not support combustion. These properties, together with its relative abundance and moderate cost compared with some other noble gases, make it especially useful as an inert shielding gas.
Chemical properties
For a long time argon was regarded as completely chemically inert. Under ordinary conditions it still behaves that way for nearly all practical purposes. Modern research has produced a small number of argon-containing species under exceptional low-temperature or energetic conditions, but argon has no broad everyday chemistry comparable with reactive elements such as chlorine or sulfur.
Where argon comes from
Commercial argon is obtained from air. Large air-separation plants cool air until it liquefies and separate its components by fractional distillation. Because the boiling points of nitrogen, argon and oxygen differ, carefully designed distillation systems can isolate high-purity argon as a useful co-product.
Uses of argon
- Welding: argon shields hot metal and the welding arc from oxygen, nitrogen and water vapour. It is widely used in TIG and MIG welding systems.
- Metal production: inert argon atmospheres protect reactive metals and molten materials during processing.
- Lighting: argon is used inside incandescent and specialised lamps because it does not readily attack hot filaments.
- Electronics and semiconductors: argon supplies inert atmospheres for crystal growth, plasma processes, sputtering and other manufacturing steps.
- Laboratories: gloveboxes, instruments and chemical operations may use argon to exclude oxygen and moisture.
- Insulated windows: argon can fill the space between panes to reduce heat transfer compared with ordinary air.
Argon isotopes
Natural terrestrial argon contains the stable isotopes ³⁶Ar, ³⁸Ar and ⁴⁰Ar. Argon-40 is by far the most abundant isotope in Earth’s atmosphere because it is continually produced over geological time by the radioactive decay of potassium-40 in rocks.
This relationship between potassium and argon is the basis of important geological dating methods, including potassium-argon and argon-argon dating. For broader isotope data, see the CleverlySmart complete isotope table.
Biological role and safety
Argon has no known essential biological role. It is not chemically toxic in the conventional sense, but it can be dangerous as a simple asphyxiant. Because argon can displace oxygen without producing a warning smell, leaks in poorly ventilated or confined spaces can create an oxygen-deficient atmosphere.
Liquid argon is cryogenic and can cause severe cold burns. Containers of compressed or liquefied argon also present pressure hazards and require appropriate handling.
Discovery and name
Argon was discovered in 1894 by Lord Rayleigh and William Ramsay while investigating a small unexplained difference between nitrogen obtained from air and nitrogen obtained from chemical compounds. They isolated a previously unknown gas from the atmosphere.
The name comes from the Greek word argos, meaning inactive or idle, reflecting its extraordinary chemical inertness.
Frequently asked questions
Is argon flammable?
No. Argon is a non-flammable noble gas and does not support ordinary combustion.
Why is argon used for welding?
It provides an inert shield that helps keep reactive atmospheric gases away from the hot weld pool and electrode.
What is argon’s electron configuration?
[Ne] 3s² 3p⁶, giving it a filled valence shell.
Argon in Earth’s atmosphere
Argon makes up about 0.93% of dry air by volume, making it the third-most abundant gas in Earth’s atmosphere after nitrogen and oxygen. Much of atmospheric argon is argon-40, produced over geological time by radioactive decay of potassium-40 in rocks.
Why argon is useful as an inert gas
Argon’s filled electron shell makes it extremely unreactive under ordinary conditions. This makes it ideal for shielding reactive materials from air. In gas-tungsten and gas-metal arc welding, argon protects the hot weld pool from oxygen and nitrogen. It is also used in metal production, semiconductor processing, laboratory glove boxes and some types of lighting.
Argon-argon and potassium-argon dating methods use the relationship between potassium-40 and argon-40 to determine the ages of volcanic rocks, making argon important in geology as well as industry.
Frequently asked questions
Is argon toxic?
Argon is not chemically toxic, but it can displace oxygen in poorly ventilated spaces and create an asphyxiation hazard.
Why is argon used in light bulbs?
Its low reactivity helps protect hot filaments and electrodes from oxidation while allowing stable electrical operation.
