Argon

1 · Identity

Symbol, number

Ar, 18

Oxidation states in water

0 only, as dissolved Ar (aq).

Note

No aqueous chemistry.

Sources

Identity

Name and symbol

Argon, Ar

Atomic number

18 protons

Position

group 18 · period 3 · p-block · noble gas

CAS number

7440-37-1

Atomic structure

Atomic mass

39.948 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶
[Ne] 3s²³p⁶

Electrons per shell

2, 8, 8

Valence electrons

8 outer shell

Physical properties

State at room temperature

Gas

Melting point

83.8 K (-189.35 °C)

Boiling point

87.3 K (-185.85 °C)

Density

0.0018 g/cm3 (gas at STP, so 1.7837 g/L)

Appearance

colorless gas exhibiting a lilac/violet glow when placed in an electric field

Thermal conductivity

17.72-3 W/(m·K)

Electrical resistivity

not in sources

Electrical conductivity

not in sources

Crystal structure

face-centered cubic

Molar heat capacity

not in sources

Chemical properties

Oxidation states

0

Electronegativity

3.242 (Allen Scale)

Ionisation energy

15.76 eV
1st 1,520.6, 2nd 2,665.8, 3rd 3,931 kJ/mol

Electron affinity

0 eV

Atomic radius

empirical 106, covalent 106, van der Waals 188 pm

Ionic radius

not in sources

Reactivity

A noble gas with a complete octet ([Ne] 3s2 3p6); very inert, with no true compound under ordinary conditions, so it serves as the standard inert blanket for welding, titanium production and crystal growth.

with water

Does not react; it merely dissolves, about as soluble as oxygen.

with oxygen, air

Does not react.

with acids

Does not react.

with halogens

Does not react with fluorine gas; the only known argon-fluorine compound is argon fluorohydride, HArF, made in 2000 by ultraviolet light on frozen argon containing hydrogen fluoride and stable only at very low temperature.

Typical compounds

• HArF argon fluorohydride first neutral argon compound (2000); exists only in cold matrix

Occurrence, production and use

Crustal abundance

3.5 milligrams per kilogram

Oceanic abundance

4.5×10-1 milligrams per liter

Occurrence and sources

The gas is prepared by fractionation of liquid air because the atmosphere contains 0.94% argon. The atmosphere of Mars contains 1.6% of 40Ar and 5 ppm of 36Ar.

• dissolved in seawater about 0.45 mg/L; crustal estimate 3.5 mg/kg (Jefferson Lab figures via PubChem)
• Ar in air 0.94 percent of the atmosphere by volume, replenished by potassium-40 decay; the only source
• Ar in the Martian atmosphere 1.6 percent argon-40 and 5 ppm argon-36, of no industrial relevance

Extraction, production

Fractional distillation of liquid air

Physical separation, no chemical reaction; argon is recovered alongside oxygen and nitrogen.

Uses

Argon is frequently used when an inert atmosphere is needed. It is used to fill incandescent and fluorescent light bulbs to prevent oxygen from corroding the hot filament. Argon is also used to form inert atmospheres for arc welding, growing semiconductor crystals and processes that require shielding from other atmospheric gases.

Once thought to be completely inert, argon is known to form at least one compound. The synthesis of argon fluorohydride (HArF) was reported by Leonid Khriachtchev, Mika Pettersson, Nino Runeberg, Jan Lundell and Markku Räsänen in August of 2000. Stable only at very low temperatures, argon fluorohydride begins to decompose once it warms above -246°C (-411°F). Because of this limitation, argon fluorohydride has no uses outside of basic scientific research.

It is used in electric light bulbs and in fluorescent tubes at a pressure of about 400 Pa. and in filling photo tubes, glow tubes, etc. Argon is also used as an inert gas shield for arc welding and cutting, as blanket for the production of titanium and other reactive elements, and as a protective atmosphere for growing silicon and germanium crystals.

• Metals and welding: inert shield gas for arc welding and cutting, a substitute for helium; blanket for titanium and other reactive metal production
• Electronics: protective atmosphere for growing silicon and germanium crystals; photo tubes and glow tubes
• Lighting: incandescent bulbs to stop the filament oxidising; fluorescent tubes and low-energy lamps with mercury
• Construction and consumer goods: double-glazing fill; tyre fill in luxury cars to protect the rubber and reduce road noise

Safety, toxicity

GHS classification, signal word Warning

• H280 Contains gas under pressure; may explode if heated Gases under pressure
• H281 Contains refrigerated gas; may cause cryogenic burns or injury Gases under pressure

Discovery and name

Discovered by

Lord Rayleigh and William Ramsay

Discovered

1894

First isolated

not in sources

Named by

not in sources

Origin of the name

from the Greek ἀργόν, meaning 'lazy' or 'inactive', in reference to its inertness

Argon is two and one half times as soluble in water as nitrogen, having about the same solubility as oxygen. Argon is colorless and odorless, both as a gas and liquid. Argon is considered to be a very inert gas and is not known to form true chemical compounds, as do krypton, xenon, and radon.