Argon

Argon
Atomic number	18
Symbol	Ar
Group	18 (noble gases)
Boiling point	-185.8 °C
Electron configuration	[Ne] 3s2 3p6
Discovery	1894 (Ramsay, Rayleigh)
Period	3
Main isotopes	36Ar, 38Ar, 40Ar
Cas number	7440-37-1
Block	p
Phase STP	Gas
Wikidata	Q696

Argon is a chemical element (symbol Ar, atomic number 18) that exists as a colorless, odorless, monatomic gas under normal conditions. It belongs to the noble gas family (Group 18 of the periodic table) and is chemically inert in ordinary environments. Argon makes up about 0.934% of Earth’s atmosphere by volume (the third most abundant gas after nitrogen and oxygen). Because of its lack of reactivity and availability, argon is widely used in applications requiring an inert atmosphere (for example, welding or metal fabrication), in gas discharge lighting, and as a cryogenic coolant.

Overview and Key Facts

Symbol and Atomic Number: Ar, 18.
Group/Period/Block: Noble gas in Group 18, Period 3, p-block of the periodic table.
Atomic Weight: ~39.948 u (unified atomic mass units).
Electron Configuration: [Ne] 3s² 3p⁶ (complete 3rd shell; 8 valence electrons).
Valence Electrons: 8 (full octet; makes argon highly stable and unreactive).
Common Oxidation State: 0 (argon's atoms do not normally form bonds; it has no true chemical compounds under ordinary conditions).
Phase at STP: Colorless, monatomic gas (STP = 0 °C, 1 atmosphere).
Abundance: ~0.93% of air by volume. Argon is the most abundant noble gas in Earth’s atmosphere.
Key Uses: Inert shielding gas for welding and metal processing; filler gas in incandescent/glow lamps and fluorescent tubes; argon-ion and excimer lasers (blue/green light sources); cryogenics (liquid argon); and other technologies requiring inert conditions.

Argon is odourless, tasteless, and non-toxic. It condenses to a colorless liquid at temperatures below 87.30 K (–185.85 °C) and freezes to a solid at 83.80 K (–189.35 °C). The element’s high first ionization energy (15.76 eV) reflects its closed-shell electron configuration and stability.

Atomic Structure and Electron Configuration

Argon’s nucleus contains 18 protons and (for the most abundant isotope) 22 neutrons. Around the nucleus are 18 electrons distributed in three shells: the first shell (1s²), second shell (2s² 2p⁶), and third shell (3s² 3p⁶). In shorthand, argon’s ground-state electron configuration is [Ne] 3s² 3p⁶, which means its outer (valence) shell is fully filled with eight electrons. We say argon has a “complete octet,” which explains why it is exceptionally stable and has no tendency to gain or lose electrons under normal conditions.

In the Periodic Table, argon sits below neon and above krypton. Moving from left to right across the third period, nuclear charge increases and atomic radius generally decreases. Argon’s atomic radius (covalent radius) is about 71 pm (0.071 nm). Compared to neighboring elements, argon has a relatively high first ionization energy (~15.76 eV, or ~1520 kJ/mol). This is because removing an electron from a filled shell is difficult. For example, argon’s first ionization energy is higher than that of its neighbors potassium and chlorine, reflecting its full valence shell. Because it does not form bonds, argon has no standard electronegativity value (it is effectively nonpolar and chemically “inert”).

As a Group 18 element, argon is often called a noble gas. Noble gases have entirely filled outer electron shells, making them stable and largely unreactive. Argon’s electron configuration is the same as potassium’s noble gas core Ar plus an extra filled p-shell, which is why potassium often loses one electron and argon loses none (potassium is easily ionized to K⁺, while Ar remains neutral).

Isotopes and Nuclear Properties

Natural argon consists mainly of three stable isotopes: argon-36 (²⁶Ar), argon-38 (³⁸Ar), and argon-40 (⁴⁰Ar). On Earth, almost all argon (~99.6%) is ⁴⁰Ar, the heaviest of these, because ⁴⁰Ar is produced by the radioactive decay of potassium-40 in Earth’s crust. The relative abundances in air are about 0.34% ³⁶Ar, 0.063% ³⁸Ar, and 99.6% ⁴⁰Ar. (In the universe as a whole or in primitive solar material, ³⁶Ar and ³⁸Ar dominate, since ⁴⁰Ar grows only by radioactive decay of potassium.)

⁴⁰Ar has a nuclear spin of 0 (even number of protons and neutrons). Likewise, ³⁶Ar and ³⁸Ar have spin 0 (they are both even-even nuclei). All three stable isotopes are non-radioactive. Among unstable (radioactive) isotopes, argon-39 (³⁹Ar) is notable: it has a half-life of about 269 years and is produced by cosmic-ray interactions in the atmosphere. ³⁹Ar decays by beta emission to ³⁹K. Because of its intermediate half-life, ³⁹Ar is used for dating timescales of decades to centuries in the Earth and atmosphere sciences (for example, dating of groundwater or polar ice). Another isotope, argon-37 (³⁷Ar), has a half-life of 35 days; it decays by electron capture and is used as a tracer in some neutrino detection experiments (since ³⁷Ar is produced by neutrino interactions in large volumes of argon). Argon-41 (⁴¹Ar) (half-life ~110 minutes) can appear briefly around nuclear reactors and is monitored as part of reactor safety.

The decay of potassium-40 (⁴⁰K) via electron capture (or positron emission) to argon-40 underlies the potassium-argon (K–Ar) dating method in geology: ⁴⁰K in minerals decays to ⁴⁰Ar over billions of years, allowing determination of rock ages. Because argon is inert, the ⁴⁰Ar formed remains trapped in solid geological materials, serving as a clock.

Allotropes and Typical Compounds

Argon does not form allotropes. It exists naturally only as single (monatomic) argon atoms. (By contrast, some elements like carbon or oxygen have multiple allotropes; argon has none.) In the gas phase, argon atoms are well separated and only interact weakly by Van der Waals (dispersion) forces when cooled to a liquid or solid. For example, very cold argon gas can show a faint association of two atoms (Ar₂) due to Van der Waals attraction, but Ar₂ is not a chemical compound (it is not bonded by shared electrons and dissociates at slightly higher temperature).

Under normal conditions, argon does not form chemical compounds with other elements – this is why it is considered inert. However, a few exotic argon compounds have been discovered under extreme conditions (often at cryogenic temperatures or in electrical discharges). The only stable neutral molecule containing argon is hydrogen fluoride argon (HArF), made at temperatures below 17 K: in a frozen mixture of HF and argon with an electric discharge, a fleeting molecule H–Ar–F can be formed. Similarly, metastable argon fluoride (ArF) and argon chloride (ArCl) complexes appear as short-lived species in plasma. Argon does form excited-state dimers called excimers (for example, Ar₂) in gas lasers; these exist only in the excited state and quickly dissociate into two argon atoms when they drop back to the ground state, emitting ultraviolet light (the ArF excimer laser, emitting at 193 nm, is widely used in semiconductor lithography).

Ionic argon-containing compounds occur in space and in ion chemistry: for example, the ion ArH⁺ (argonium hydride) has been detected in the interstellar medium. Chemically, argon has also been found in clathrate-like “inclusion” compounds: argon atoms can become trapped in cage-like structures of water or organic crystals (argon clathrate hydrates), but in these the argon is merely physically trapped, not chemically bound. In short, argon’s typical “compounds” are extremely limited and require special conditions; it has virtually no conventional oxide, hydride, or halide at room temperature and pressure.

Physical Properties

Argon is a monatomic gas under standard conditions. It is colorless, odorless, and tasteless, with very low chemical reactivity. Key physical constants include:

Density (gas at STP): 1.784 g/L (0.001784 g/cm³) at 0 °C and 1 atm (roughly 1.784 kg/m³). (For comparison, air is about 1.225 kg/m³, so argon is slightly denser than air.)
Density (liquid): about 1.40 g/cm³ (1400 kg/m³) at its boiling point.
Crystal (solid) density: ~1.65 g/cm³ at 84 K (solid argon is an fcc (face-centered cubic) crystal, space group Fm3̅m, with a lattice constant ≈525.6 pm).

Melting (fusion) point: 83.8058 K (–189.344 °C).
Boiling point: 87.302 K (–185.848 °C).
Critical point: 150.86 K, 48.04 atm.
Triple point: 83.805 K at 0.687 atm.

As a solid or liquid, argon is colorless (no visible color), and it does not make magnetic fields (it is diamagnetic). Its thermal conductivity is low: roughly 0.0177 W·m⁻¹·K⁻¹ at room temperature (typical for a heavy monatomic gas). Argon is an electrical insulator. In high-voltage equipment, argon does not conduct unless it is ionized into a plasma; in a gas discharge tube, argon produces characteristic spectral lines. When excited by an electric discharge, argon glows with a pale lavender or blue color. Prominent emission lines occur at 696.5 nm (red) and 763.5 nm (deep red) among others, which are in fact visible in argon-neon lamps and in the output of argon-ion lasers (blue-green lines at 488 nm, 514 nm, etc., are from Ar⁺ ions). These spectral lines make argon useful in analytical spectroscopy (argon discharge lamps serve as wavelength calibration sources).

Because argon atoms are spherical and non-polar, liquid argon’s viscosity and surface tension are relatively low; it boils and freezes sharply at its boiling and melting points. Its coefficient of thermal expansion is large near the boiling point (as with any gas turning liquid). In technological systems (cryostats), argon’s physical properties – especially its low boiling point and high density as liquid – make it a good refrigerant for certain applications.

Chemical Reactivity and Trends

Argon’s chemical behavior is dominated by its inertness. With a full valence shell, an argon atom has no tendency to gain, lose, or share electrons. As a result, under normal conditions argon does not react with acids, bases, or most other chemicals. It does not burn, does not oxidize materials, and does not form compounds with oxygen or hydrogen under ambient conditions.

In practical terms, argon is often used to provide an inert (chemically non-reactive) atmosphere. For example, argon gas is used to shield hot metals during welding or cutting, preventing oxidation of the metal. It is also used in the production and handling of reactive metals (like titanium or uranium); argon blankets prevent unwanted reactions with air. In electronics and glassmaking, argon prevents fires and unwanted chemistry by excluding oxygen.

In the context of the “reactivity series” of elements, argon effectively sits at the very bottom: it is less reactive than even the noble metals and halogens. It shows virtually no tendency to be involved in acid-base or redox chemistry. Aqueous solutions of argon are simply water with dissolved argon (no effect chemically).

At very high energies or isotope levels, argon can form unusual compounds, but only with strong energy input (laser discharges, nuclear processes). In chemical terms, one could say argon’s effective oxidation state is 0 for almost all conceivable conditions. (Chemists sometimes mention hypothetical +1 or +2 oxidation states for excited argon or argon bound to a superelectrophile, but these are exceptions under extreme conditions.)

Because argon does not corrode or passivate surfaces, it is often chosen for inert gas filling in light bulbs and electronic tubes. It does not react with filament materials, glass, or phosphors. Argon’s lack of reactivity also means it is not part of any usual chemical cycles or buffers in nature.

Occurrence and Production

Argon is relatively abundant on Earth. As noted, it constitutes about 0.93% of the atmosphere by volume (and about 1.28% by weight). This makes it the third most common gas in air (after nitrogen ~78% and oxygen ~21%). Most of this atmospheric argon is isotope ⁴⁰Ar. Argon is also found trapped in natural gas and in some aquifers dissolved in water. In mineral deposits, argon can be occluded (trapped) in the crystalline lattices of rocks, especially where potassium feldspar minerals are abundant (since ⁴⁰Ar is produced there by K-40 decay).

In the universe at large, argon is less common than on Earth. Helium, hydrogen, oxygen, carbon, and others are more abundant. In cosmic abundance, argon ranks around 12th among elements (relative to hydrogen), and it exists mostly as ³⁶Ar and ³⁸Ar (the “primordial” isotopes) in stars and nebulae. On other planetary bodies, argon can be present: for example, Mars’s atmosphere contains a few percent argon (mostly ³⁶Ar and ³⁸Ar).

Production: Industrial argon is not mined or extracted from ores (because it is a noble gas and rarely forms minerals). Instead, argon is produced as a byproduct of air separation. In air distillation plants (which liquefy air and fractionally distill it to obtain oxygen and nitrogen), argon is typically separated out. Since argon boils between oxygen and nitrogen, a stream concentrated in argon is collected and purified in separate cryogenic columns. Thus major producers of argon are the same industrial gas companies that produce oxygen and nitrogen (for example, The Linde Group, Air Liquide, Praxair/Linde, Air Products & Chemicals, and others). Argon is sold in compressed gas cylinders and as a liquid at cryogenic temperatures.

Worldwide, argon production (in tonnes or in cubic meters of gas per year) is large due to its many industrial uses, but exact figures vary. Many countries with steel, electronics, and automaking industries have significant argon supply. The United States, China, and European countries are among the largest producers (since they have many air separation plants). There is no single “argon ore” – the element is obtained essentially for free when air is liquefied for other gases.

Applications and Technology

Argon’s principal roles derive from its inertness. Its largest volume use is as a shielding or cover gas. In welding (both electric-arc ‘TIG’ welding and inert gas metal arc welding), argon gas protects molten metal from reacting with oxygen and nitrogen. It is also used in metal fabrication furnaces (annealing, brazing, sintering) where a non-reactive atmosphere is needed. In the production of reactive metals (like titanium, zirconium, and uranium), argon atmospheres prevent explosive reactions with air or moisture.

In lighting technology, argon fills inert-gas lamps. Ordinary incandescent light bulbs are often filled with argon (sometimes mixed with nitrogen) to reduce evaporation of the tungsten filament and to improve efficiency compared to a vacuum. Gas discharge lamps (like fluorescent tubes and neon lamps) often contain argon: an electric discharge through a low-pressure argon gas produces a glow; phosphors on the tube walls then convert the ultraviolet/blue emission into visible light. The distinctive blue-green glow of neon signs actually often comes from argon mixed with mercury vapor. Argon is also used in plasma screens and in devices like Geiger counters or spectrographs as the host gas.

Argon is widely used in laser technology. An argon-ion laser (discovered in the 1960s) is a high-power continuous-wave laser that emits blue and green light (common lines at 488.0 nm and 514.5 nm). These lasers have been used in medicine (e.g. for retinal surgery), in research, and in entertainment (light shows). Another important laser is the argon-fluoride (ArF) excimer laser, which emits deep ultraviolet light at 193 nm. ArF lasers are crucial in photolithography for fabricating computer chips, as they enable very fine features. More recently, argon is used in inert gas lasers and in advanced imaging technologies.

In the field of cryogenics and particle physics, liquid argon plays a major role. Because argon liquefies at 87 K, it is a convenient cryogen for cooling applications and insulators. Large volumes of liquid argon are used in neutrino detectors and dark-matter experiments (e.g. the DUNE experiment, ICARUS, DarkSide) because liquid argon is a good scintillator (it produces flashes of light when a charged particle passes through) and allows the detection of rare particle events with high precision. Its density is about 1.4 g/cm³, making it practical to store in large cryostats. Liquid argon is also used in industrial freezers for food preservation and in cryopreservation.

Other applications include:

Semiconductor fabrication: Argon plasma is used for sputter etching and deposition in chip manufacturing. In reactive-ion etching, argon ions physically sputter material from wafers.
Analytical instruments: Argon is the plasma gas in inductively coupled plasma mass spectrometry (ICP-MS) and atomic emission spectroscopy, where it provides an inert, high-temperature plasma.
Metrology: Because of its inertness, argon is used as a calibration gas for instruments. Its known spectral lines calibrate spectroscopic equipment.
Fire suppression: In some fire extinguisher systems (e.g. Halon replacements), argon mixed with nitrogen can be used as a clean fire-suppressant gas.
Deep-sea diving: Special gas mixes for deep diving sometimes use argon (or a helium-argon mixture) because argon has a narcotic effect on the body at high pressure (similar to nitrogen narcosis). Although helium is more common, argon is used in some saturation diving gases due to its thermal properties, even though it induces narcosis faster than nitrogen.
Others: Argon is also infused in certain wine bottles to displace oxygen (preserving wine longer), and is occasionally used as a working fluid in specialized engines or in the glass industry to prevent oxygen contamination.

Overall, argon’s combination of low cost (since it’s a byproduct of air) and chemical inertness makes it a versatile industrial gas.

Biology, Environment, and Safety

Biologically and environmentally, argon is essentially harmless. It has no known role in metabolism or life processes. In air, argon is only about one percent, and organisms neither absorb nor utilize it (it passes through the lungs unchanged). Argon is non-toxic in the chemical sense: it does not react with tissues or disrupt biochemical reactions. However, like other inert gases, it can act as a simple asphyxiant if it displaces oxygen. Breathing pure argon (or an argon-rich mixture) can cause unconsciousness or suffocation due to oxygen deprivation, since it does not provide oxygen for respiration. In confined spaces or industrial settings, high concentrations of argon can lower the oxygen level to dangerous values. Therefore, safety guidelines require monitoring oxygen levels where argon (or nitrogen, helium, etc.) is used in enclosed areas.

Argon is colorless and odorless, so exposure is easy to overlook: special alarms or detectors for low oxygen are the common protective measures. Exposure limits for argon are essentially tied to those for inert gases generally; for example, safety regulations often say air with less than ~19.5% oxygen is unsafe (since argon or nitrogen may be high). There is no specific permissible exposure limit (PEL) for argon by itself, but OSHA and other agencies emphasize adequate ventilation whenever using it (to prevent hypoxia).

At high pressures (such as in deep diving), argon can have narcotic effects: it contributes to inert gas narcosis similar to nitrogen. Its narcotic potency is somewhat greater than nitrogen’s. Decompression tables for argon diving do not exist in common practice (helium believes), but divers acknowledge argon’s narcotic effect.

Environmentally, argon is inert and does not accumulate or react. It does not contribute to greenhouse warming or ozone depletion, and it is not considered a pollutant. Its natural abundance in the atmosphere is stable and well-mixed worldwide (the tiny anthropogenic contributions of argon are negligible). If released, argon simply rejoins the atmosphere.

In medical research, argon is being investigated for possible therapeutic effects. Some experimental studies suggest that inhaling very small amounts of argon (often mixed with other gases) might have neuroprotective effects after brain injury or cardiac arrest. However, these findings are preliminary and not yet an approved medical application. There is currently no recognized nutritional or pharmacological role for argon in humans or animals.

In summary, argon is extremely safe to handle under normal conditions. Precautions focus on asphyxiation risk (proper ventilation, no enclosed spaces filled with argon) and handling the liquid under cryogenic safety protocols (protective equipment for low temperatures, pressure relief on vessels). Otherwise, argon is not corrosive and does not cause burns or chemical damage.

History and Etymology

Argon was discovered in 1894 by Lord Rayleigh (John William Strutt) and Sir William Ramsay in London. They found that nitrogen gas extracted from air was about 0.5% denser than nitrogen from chemical compounds, indicating another heavier gas present in air. By removing oxygen and nitrogen from air, they isolated this residual gas. Analysis of its spectrum showed new lines, confirming a new element. Ramsay and Lord Rayleigh announced the discovery of the first “noble gas” on Earth (helium had been observed spectroscopically in the Sun’s spectrum earlier, but this was the first isolation from terrestrial sources).

The name “argon” was proposed by physicist Ernest Rutherford (and adopted by others) from the Greek word ἄργον (argos), meaning “inactive” or “lazy,” a reference to the element’s chemical inertness. Lord Rayleigh and Ramsay thus named the gas argon (Rayleigh later said it was also essentially inert in cost of doing nothing). The Greek root *argos is cognate with “argue,” but in this context it meant “idle.”

Argon’s discovery had interesting antecedents: in 1785, chemist Henry Cavendish had decomposed water to make “fixed air” (nitrogen) and noted that there might be a small inert component, but his results were forgotten. Rayleigh’s critical insight came around 1892–1894 when he carefully measured gas densities. In June 1894, Rayleigh sent a sample of what he thought was nitrogen to William Ramsay, who confirmed it was a new element. The formal announcement was made in August 1894.

Following its discovery, argon quickly found uses: Thomas Edison in 1896 patented a process of using argon to fill incandescent light bulbs (to protect the filament and improve efficiency). Carbide and Welsbach used argon in metal filament lamps. Throughout the 20th century, argon’s market grew with the development of new technologies requiring inert environments. In 1960, Mao and Bell demonstrated the emplacement of argon as excited gas, leading to argon-ion lasers. In recent decades, the use of argon in scientific instruments (ICP-MS, lasers) and particle detectors has become important.

Potassium–argon dating (developed in the mid-20th century) used argon isotopes to date volcanic rocks; this was a milestone in geology. The first dating of a rock by the K–Ar method (1948) was done by George Tilton and William Dalrymple.

As for etymology: the symbol Ar derives from “argon.” The element has no alternative name in English; related languages use similar forms (e.g. German Argon, French argon). The adjective is “argonaceous” (rarely used).

Over time, argon’s image as a “lazy” or “noble” gas has remained apt: it is remarkably unaffected by changes in chemistry or environment.

Data Table

Property	Value
Symbol	Ar
Atomic number (Z)	18
Atomic weight	39.948 u (≈6.633×10⁻²³ g)
Electron configuration	[Ne] 3s² 3p⁶
Valence electrons	8
Group	18 (noble gases)
Period	3
Block	p-block
Phase at STP	Gas
Density (gas, STP)	1.784 kg·m⁻³ (0 °C, 1 atm)
Density (liquid)	~1.40×10³ kg·m⁻³ (at boiling point)
Melting point	83.80 K (–189.35 °C)
Boiling point	87.30 K (–185.85 °C)
Heat of fusion	1.188 kJ·mol⁻¹
Heat of vaporization	6.447 kJ·mol⁻¹
Thermal conductivity	~0.0177 W·m⁻¹·K⁻¹ (gas at 300 K)
Specific heat (Cp)	0.520 J·g⁻¹·K⁻¹ (ideal gas, 300 K)
Crystal structure	Face-centered cubic (fcc)
Atomic radius (covalent)	71 pm (0.071 nm)
Van der Waals radius	188 pm
First ionization energy	1520.6 kJ·mol⁻¹ (15.76 eV)
Electron affinity	(none; argon does not gain electrons)
Oxidation state(s)	0 (virtually exclusively; very rare +ک under extreme conditions)
Isotopes (stable)	^36Ar, ^38Ar, ^40Ar (40Ar ≈99.6% on Earth)
Discovery	1894, by Lord Rayleigh (Strutt) and W. Ramsay
Name origin	Greek argos = “inactive, idle” (reflecting its inertness)