Argon – facts and properties
- Symbol
- Ar
- Atomic number
- 18
- Atomic mass
- 39.948 u
- Period
- 3
- Group
- 18
- Category
- noble gas
- Electron configuration
- [Ne] 3s² 3p⁶
- Atomic radius
- 71 pm
- Valence electrons
- 8
- State at standard conditions
- gas
- Melting point
- -189.3 °C
- Boiling point
- -185.8 °C
- Density
- 1.78 g/L
- Discovered
- 1894
Argon is a colourless, odourless noble gas with atomic number 18. It is the most abundant noble gas in the atmosphere: nearly one percent of every breath you take is argon, far more than carbon dioxide. Its name comes from the Greek word for lazy, because argon hardly ever forms compounds. That very lack of reactivity is what makes it useful, as a shielding gas in welding, inside light bulbs and between the panes of double glazing.
Position in the periodic table
Occurrence and extraction
After nitrogen and oxygen, argon is the third most common component of dry air. Most of it has come out of the rocks of the Earth. Potassium-40, a naturally radioactive isotope of potassium, slowly decays, partly into argon-40, and over billions of years this gas has built up in the atmosphere.
Argon was discovered in 1894 by Lord Rayleigh and William Ramsay. Rayleigh had noticed that nitrogen taken from the air was always slightly denser than nitrogen made from chemical compounds. Ramsay removed the nitrogen from a sample of air by letting it react with red-hot magnesium, and a small amount of an unknown gas was left over. Both men later received Nobel Prizes for this work, Rayleigh in physics and Ramsay in chemistry. Argon was the first noble gas to be isolated, and the others followed within a few years.
Today almost all argon is obtained from air. In air separation plants, air is cooled until it liquefies and is then distilled. Because the boiling point of argon lies between those of nitrogen and oxygen, it can be drawn off as a separate fraction and purified further. Argon is therefore a by-product of producing oxygen and nitrogen.
Properties and reactions
Argon has the electron configuration [Ne] 3s² 3p⁶. Its outer shell is completely filled with eight electrons, an octet that is particularly stable. Argon belongs to period 3 and group 18, the noble gases. It neither gives up nor accepts electrons easily, so under ordinary conditions it forms no chemical compounds at all. Only at extremely low temperatures have researchers managed to make an unstable compound of argon, hydrogen and fluorine.
Argon exists as single atoms rather than molecules. With a density of 1.78 g/L it is heavier than air. When cooled it turns liquid at -185.8 °C and solid at -189.3 °C, so at normal pressure liquid argon exists only within a narrow temperature range. In a gas discharge tube argon glows violet. It also conducts heat less well than air.
Argon is neither toxic nor flammable, but it can still be dangerous. Being heavier than air, it can collect in cellars, pits and tanks and push out the oxygen there. Since it can be neither seen nor smelled, nobody notices, and there is a risk of suffocation.
Uses in everyday life and technology
The metal industry uses most of the argon produced. In gas-shielded welding, argon surrounds the weld and keeps out oxygen and nitrogen, which would otherwise attack the hot metal. Aluminium and stainless steel in particular are welded this way. Steelworks bubble argon through molten steel to stir it and remove dissolved gases, and in stainless steel production a mixture of argon and oxygen lowers the carbon content without burning off too much chromium. Argon also protects titanium during its production and shields the silicon crystals grown for computer chips from unwanted reactions.
At home you will find argon in double and triple glazing. The gap between the panes is often filled with argon, which conducts heat less well than air and so saves heating energy. Incandescent bulbs contain argon because the gas slows the evaporation of the filament, and fluorescent tubes contain it too. Some foods are packed under argon as a protective gas, and some divers inflate their drysuits with argon because it insulates better than air.
In medicine and research, argon lasers and argon plasma are used, for example in eye surgery and to stop bleeding. Geologists use the decay of potassium into argon to work out the age of rocks.
Quick question
Which element has the electron configuration [Ar] 4s1?
FAQ
Is argon dangerous to breathe?
Argon is not poisonous and does not react in the body. It becomes dangerous only in large amounts in enclosed spaces, where it displaces oxygen. Because argon has no smell, none of our senses warns us, so workplaces that use a lot of argon commonly install oxygen monitors.
Why is argon unreactive?
Argon has a completely filled outer shell of eight electrons. To form a bond it would have to lose, gain or share electrons, and that costs a lot of energy. Argon therefore stays as single atoms and forms no compounds under normal conditions.
What is argon used for?
Mostly as a shielding gas in welding and metal production. It also fills insulated windows and light bulbs, protects packaged food and is needed in the manufacture of semiconductors.