Ar 18

Argon (Ar)

noble-gas

Period: 3 Group: 18 Block: p

Gas

Standard Atomic Weight

Electron configuration

[Ne] 3s² 3p⁶

Melting point

-189.35 °C (83.8 K)

Boiling point

-185.85 °C (87.3 K)

Density

1.7837 kg/m³

Oxidation states

Electronegativity (Pauling)

N/A

Ionization energy (1st)

Discovery year

1894

Atomic radius

71 pm

Details

Name origin Greek: argos (inactive).

Discovery country Scotland

Discoverers Sir William Ramsey, Baron Rayleigh

Argon is a colorless noble gas and the third most abundant gas in Earth’s atmosphere, after nitrogen and oxygen. Its closed-shell electron configuration makes it chemically very inert under ordinary conditions. Most terrestrial argon is ⁴⁰Ar, produced by the radioactive decay of ⁴⁰K in rocks. The element is valued mainly as a dense, nonreactive atmosphere for industrial, analytical, and lighting applications.

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.

The name derives from the Greek argos for "lazy" or "inactive" because it does not combine with other elements. It was discovered in 1894 by the Scottish chemist William Ramsay and the English physicist Robert John Strutt (Lord Rayleigh) in liquefied air. Rayleigh's initial interest derived from a problem posed by the English physicist Henry Cavendish in 1785, i.e., when oxygen and nitrogen were removed from air, there was an unknown residual gas remaining.

Argon was discovered by Sir William Ramsay, a Scottish chemist, and Lord Rayleigh, an English chemist, in 1894. Argon makes up 0.93% of the earth's atmosphere, making it the third most abundant gas. Argon is obtained from the air as a byproduct of the production of oxygen and nitrogen.

From the Greek argos, inactive. Its presence in air was suspected by Cavendish in 1785, discovered by Lord Raleigh and Sir William Ramsay in 1894.

Read about Argon on Wikipedia

Images

Properties

Physical

Atomic radius (empirical) 71 pm

Covalent radius 106 pm

Van der Waals radius 188 pm

Density

Molar volume 0.0242 L/mol

Phase at STP gas

Melting point -189.35 °C

Boiling point -185.85 °C

Thermal conductivity 0.018 W/(m·K)

Specific heat capacity 0.52 J/(g·K)

Molar heat capacity 20.786 J/(mol·K)

Crystal structure fcc

Chemical

Electronegativity (Allen) 3.242

Electron affinity

Ionization energy (1st)

Ionization energy (2nd)

Ionization energy (3rd)

Ionization energy (4th)

Ionization energy (5th)

Oxidation states 0

Valence electrons 8

Electron configuration

Electron configuration (semantic)

Thermodynamic

Triple point (temperature) -189.34 °C

Triple point (pressure) 6.890000e+4 Pa

Critical point (temperature) -122.463 °C

Critical point (pressure) 4.863000e+6 Pa

Heat of fusion 0.01222988 eV

Heat of vaporization 0.06664248 eV

Heat of atomization 0 eV

Nuclear

Stable isotopes 3

Discovery year 1894

Abundance

Abundance (Earth's crust) 3.5 mg/kg

Abundance (ocean)

Reactivity

N/A

Crystal Structure

Lattice constant a 526 pm

Electronic Structure

Electrons per shell 2, 8, 8

Identifiers

CAS number 7440-37-1

Term symbol

InChI InChI=1S/Ar

InChI Key XKRFYHLGVUSROY-UHFFFAOYSA-N

Electron Configuration Measured

Ion charge

Protons 18

Electrons 18

Charge Neutral

Configuration Ar: 3s² 3p⁶

[Ne] 3s² 3p⁶

1s² 2s² 2p⁶ 3s² 3p⁶

Orbital diagram

↑↓

2/2

↑↓

2/2

↑↓

6/6

↑↓

2/2

↑↓

6/6

Total electrons: 18 Unpaired: 0

Atomic model

Protons 18

Neutrons 22

Electrons 18

Mass number 40

Stability Stable

Isotopes change neutron count, mass, and stability — not the electron configuration of a neutral atom.

Schematic atomic model, not to scale.

Atomic Fingerprint

Emission / Absorption Spectrum

Emission Visible: 380–750 nm

Isotope Distribution

Mass number	Atomic mass (u)	Natural abundance	Half-life
36 Stable	35.967545105 ± 0.000000028	0.3336%	Stable
38 Stable	37.96273211 ± 0.00000021	0.0629%	Stable
40 Stable	39.9623831237 ± 0.0000000024	99.6035%	Stable

Measured

Phase / State

Gas 25 °C (298.15 K)

Reason: 210.8 °C above boiling point (-185.85 °C)

Melting point -189.35 °C

Boiling point -185.85 °C

Above boiling by 210.8 °C

0 K Current temperature: 25 °C 6000 K

Phase timeline

Schematic, not to scale

Solid

Liquid

Gas

Melting

Boiling

25°C

Solid

Liquid

Gas

Current

Phase transition points

Melting point Literature

-189.35 °C

Boiling point Literature

-185.85 °C

Current phase Calculated
Gas

Transition energies

Heat of fusion Literature

0.01222988 eV

Energy required to melt 1 mol at melting point

Heat of vaporization Literature

0.06664248 eV

Energy required to vaporize 1 mol at boiling point

Density

Reference density Literature

1.7837 kg/m³

At standard conditions

Current density Estimated

1.632836 kg/m³

Estimated via ideal gas law at current T

Advanced

Triple point Literature

-189.34 °C

Critical point Literature

-122.463 °C

Atomic Spectra

Showing 10 of 18 Atomic Spectra. Sorted by ion charge (ascending).

Lines Holdings ?

Ion	Charge	Total lines	Transition probabilities	Level designations
Ar I	0	461	428	429
Ar II	+1	2858	307	2858
Ar III	+2	509	80	509
Ar IV	+3	256	42	256
Ar V	+4	111	18	111
Ar VI	+5	104	6	104
Ar VII	+6	218	21	218
Ar VIII	+7	141	27	141
Ar IX	+8	178	2	178
Ar X	+9	92	1	92

NIST Lines Holdings →

Levels Holdings ?

Ion	Charge	Levels
Ar I	0	504
Ar II	+1	419
Ar III	+2	125
Ar IV	+3	58
Ar V	+4	49
Ar VI	+5	44
Ar VII	+6	95
Ar VIII	+7	72
Ar IX	+8	98
Ar X	+9	71

NIST Levels Holdings →

18 Ar 39.948

Argon — Atomic Orbital Visualizer

[Ne]3s23p6

Energy levels 2 8 8

Oxidation states 0

HOMO 3p n=3 · l=1 · m=-1

Argon — Atomic Orbital Visualizer Preview

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18 Ar 39.948

Argon — Crystal Structure Visualizer

Face-Centered Cubic · Pearson cF4

Experimental

Pearson cF4

Coord. № 12

Packing 74.000%

No crystal structure at standard conditions — gas at 298 K, 1 atm

Solid phase structure at 293 K

Argon — Crystal Structure Visualizer Preview

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Compounds

39.900 u

40.965 u

38.964 u

39.962 u

35.968 u

36.967 u

37.963 u

Isotopes (3)

Naturally occurring argon is a mixture of three isotopes. Twelve other radioactive isotopes are known to exist.

Mass number	Atomic mass (u)	Natural abundance	Half-life	Decay mode
36 Stable	35.967545105 ± 0.000000028	0.3336% ± 0.0021%	Stable	stable
38 Stable	37.96273211 ± 0.00000021	0.0629% ± 0.0007%	Stable	stable
40 Stable	39.9623831237 ± 0.0000000024	99.6035% ± 0.0025%	Stable	stable

36 Stable

Atomic mass (u) 35.967545105 ± 0.000000028

Natural abundance 0.3336% ± 0.0021%

Half-life Stable

Decay mode

stable

38 Stable

Atomic mass (u) 37.96273211 ± 0.00000021

Natural abundance 0.0629% ± 0.0007%

Half-life Stable

Decay mode

stable

40 Stable

Atomic mass (u) 39.9623831237 ± 0.0000000024

Natural abundance 99.6035% ± 0.0025%

Half-life Stable

Decay mode

stable

Spectral Lines

Showing 50 of 1065 Spectral Lines. Only spectral lines with measured intensity are shown by default.

Wavelength (nm)	Intensity	Ion stage	Type	Transition	Accuracy	Source
458.989759 nm	25704	Ar II	emission	3s2.3p4.(1D).4s 2D → 3s2.3p4.(1D).4p 2F*	Measured	NIST
472.686807 nm	23442	Ar II	emission	3s2.3p4.(3P).4s 2P → 3s2.3p4.(3P).4p 2D*	Measured	NIST
696.543 nm	10000	Ar I	emission	3s2.3p5.(2P<3/2>).4s 2[3/2] → 3s2.3p5.(2P*<1/2>).4p 2[1/2]	Measured	NIST
706.72175 nm	10000	Ar I	emission	3s2.3p5.(2P<3/2>).4s 2[3/2] → 3s2.3p5.(2P*<1/2>).4p 2[3/2]	Measured	NIST
738.39801 nm	10000	Ar I	emission	3s2.3p5.(2P<3/2>).4s 2[3/2] → 3s2.3p5.(2P*<1/2>).4p 2[3/2]	Measured	NIST
440.098598 nm	8710	Ar II	emission	3s2.3p4.(3P).3d 4D → 3s2.3p4.(3P).4p 4P*	Measured	NIST
501.716264 nm	7413	Ar II	emission	3s2.3p4.(3P).3d 2D → 3s2.3p4.(1D).4p 2F*	Measured	NIST
476.486444 nm	2344	Ar II	emission	3s2.3p4.(3P).4s 2P → 3s2.3p4.(3P).4p 2P*	Measured	NIST
460.956692 nm	2291	Ar II	emission	3s2.3p4.(1D).4s 2D → 3s2.3p4.(1D).4p 2F*	Measured	NIST
487.986345 nm	2239	Ar II	emission	3s2.3p4.(3P).4s 2P → 3s2.3p4.(3P).4p 2D*	Measured	NIST
727.29354 nm	2000	Ar I	emission	3s2.3p5.(2P<3/2>).4s 2[3/2] → 3s2.3p5.(2P*<1/2>).4p 2[1/2]	Measured	NIST
427.752786 nm	1995	Ar II	emission	3s2.3p4.(1D).4s 2D → 3s2.3p4.(1D).4p 2P*	Measured	NIST
434.806354 nm	1995	Ar II	emission	3s2.3p4.(3P).4s 4P → 3s2.3p4.(3P).4p 4D*	Measured	NIST
480.602014 nm	1820	Ar II	emission	3s2.3p4.(3P).4s 4P → 3s2.3p4.(3P).4p 4P*	Measured	NIST
454.505166 nm	1738	Ar II	emission	3s2.3p4.(3P).4s 2P → 3s2.3p4.(3P).4p 2P*	Measured	NIST
442.60008 nm	1514	Ar II	emission	3s2.3p4.(3P).4s 4P → 3s2.3p4.(3P).4p 4D*	Measured	NIST
465.79009 nm	1445	Ar II	emission	3s2.3p4.(3P).4s 2P → 3s2.3p4.(3P).4p 2P*	Measured	NIST
473.590548 nm	1000	Ar II	emission	3s2.3p4.(3P).4s 4P → 3s2.3p4.(3P).4p 4P*	Measured	NIST
714.7041 nm	1000	Ar I	emission	3s2.3p5.(2P<3/2>).4s 2[3/2] → 3s2.3p5.(2P*<1/2>).4p 2[3/2]	Measured	NIST
413.172327 nm	891	Ar II	emission	3s2.3p4.(1D).4s 2D → 3s2.3p4.(1D).4p 2P*	Measured	NIST
496.507942 nm	891	Ar II	emission	3s2.3p4.(3P).4s 2P → 3s2.3p4.(3P).4p 2D*	Measured	NIST
457.934934 nm	871	Ar II	emission	3s2.3p4.(3P).4s 2P → 3s2.3p4.(3P).4p 2S*	Measured	NIST
484.780955 nm	832	Ar II	emission	3s2.3p4.(3P).4s 4P → 3s2.3p4.(3P).4p 4P*	Measured	NIST
437.988058 nm	794	Ar II	emission	3s2.3p4.(3P).4p 2S* → 3s2.3p4.(3P).5s 2P	Measured	NIST
407.200431 nm	708	Ar II	emission	3s2.3p4.(1D).4s 2D → 3s2.3p4.(1D).4p 2D*	Measured	NIST
443.018862 nm	661	Ar II	emission	3s2.3p4.(3P).4s 4P → 3s2.3p4.(3P).4p 4D*	Measured	NIST
448.181045 nm	646	Ar II	emission	3s2.3p4.(3P).3d 2D → 3s2.3p4.(1D).4p 2D*	Measured	NIST
437.075295 nm	617	Ar II	emission	3s2.3p4.(3P).3d 2D → 3s2.3p4.(1D).4p 2D*	Measured	NIST
433.119915 nm	603	Ar II	emission	3s2.3p4.(3P).4s 4P → 3s2.3p4.(3P).4p 4D*	Measured	NIST
437.966649 nm	550	Ar II	emission	3s2.3p4.(3P).4s 4P → 3s2.3p4.(3P).4p 4D*	Measured	NIST
611.492319 nm	537	Ar II	emission	3s2.3p4.(1D).3d 2G → 3s2.3p4.(1D).4p 2F*	Measured	NIST
410.391181 nm	447	Ar II	emission	3s2.3p4.(3P).4p 4D* → 3s2.3p4.(3P).5s 4P	Measured	NIST
617.227751 nm	407	Ar II	emission	3s2.3p4.(1D).3d 2G → 3s2.3p4.(1D).4p 2F*	Measured	NIST
415.85907 nm	400	Ar I	emission	3s2.3p5.(2P<3/2>).4s 2[3/2] → 3s2.3p5.(2P*<3/2>).5p 2[3/2]	Measured	NIST
420.067472 nm	400	Ar I	emission	3s2.3p5.(2P<3/2>).4s 2[3/2] → 3s2.3p5.(2P*<3/2>).5p 2[5/2]	Measured	NIST
506.203703 nm	398	Ar II	emission	3s2.3p4.(3P).4s 4P → 3s2.3p4.(3P).4p 4P*	Measured	NIST
437.132854 nm	355	Ar II	emission	3s2.3p4.(3P).3d 4D → 3s2.3p4.(3P).4p 4P*	Measured	NIST
500.93342 nm	355	Ar II	emission	3s2.3p4.(3P).4s 4P → 3s2.3p4.(3P).4p 4P*	Measured	NIST
447.475916 nm	347	Ar II	emission	3s2.3p4.(3P).3d 2D → 3s2.3p4.(1D).4p 2P*	Measured	NIST
422.815775 nm	331	Ar II	emission	3s2.3p4.(3P).4s 4P → 3s2.3p4.(3P).4p 2D*	Measured	NIST
404.289342 nm	288	Ar II	emission	3s2.3p4.(1D).4s 2D → 3s2.3p4.(1D).4p 2D*	Measured	NIST
426.652661 nm	288	Ar II	emission	3s2.3p4.(3P).4s 4P → 3s2.3p4.(3P).4p 4D*	Measured	NIST
488.904194 nm	288	Ar II	emission	3s2.3p4.(3P).4s 2P → 3s2.3p4.(3P).4p 2P*	Measured	NIST
423.721956 nm	269	Ar II	emission	3s2.3p4.(1D).4s 2D → 3s2.3p4.(1D).4p 2P*	Measured	NIST
664.369734 nm	269	Ar II	emission	3s2.3p4.(3P).3d 4F → 3s2.3p4.(3P).4p 4D*	Measured	NIST
385.058079 nm	263	Ar II	emission	3s2.3p4.(3P).4s 4P → 3s2.3p4.(3P).4p 4S*	Measured	NIST
440.009637 nm	257	Ar II	emission	3s2.3p4.(3P).3d 4D → 3s2.3p4.(3P).4p 4P*	Measured	NIST
443.099589 nm	251	Ar II	emission	3s2.3p4.(3P).3d 4D → 3s2.3p4.(3P).4p 4P*	Measured	NIST
493.320891 nm	251	Ar II	emission	3s2.3p4.(3P).4s 4P → 3s2.3p4.(3P).4p 4P*	Measured	NIST
514.17826 nm	224	Ar II	emission	3s2.3p4.(3P).3d 2D → 3s2.3p4.(1D).4p 2F*	Measured	NIST

Extended Properties

Covalent Radii (Extended)

Covalent radius (Pyykkö)

Covalent radius (Pyykkö, double)

Covalent radius (Pyykkö, triple)

Van der Waals Radii

Bondi

Alvarez

UFF

MM3

Atomic & Metallic Radii

Atomic radius (Rahm)

Numbering Scales

Mendeleev

Pettifor

Glawe

Electronegativity Scales

Ghosh

Gunnarsson–Lundqvist

Robles–Bartolotti

Polarizability & Dispersion

Dipole polarizability

Dipole polarizability (unc.)

C₆

C₆ (Gould–Bučko)

Chemical Affinity

Proton affinity

Gas basicity

Noble Gas Properties

Density (25 °C) 1.633 g/L

Reactions

Phase Transitions & Allotropes

Melting point	83.81 K
Boiling point	87.3 K
Critical point (temperature)	150.69 K
Critical point (pressure)	4.86 MPa
Triple point (temperature)	83.81 K
Triple point (pressure)	68.89 kPa

Advanced Reference Data

Screening Constants (5)

n	Orbital	σ
1	s	0.4925
2	p	3.9918
2	s	5.7696
3	p	11.2359
3	s	10.2432

Isotope Decay Modes (46)

Isotope	Mode	Intensity
29	2p	100%
30	2p	100%
31	B+	100%
31	B+p	68.3%
31	2p	9%
31	B+pA	0.4%
31	3p	0.1%
31	B+A	0%
31	2p	0%
32	B+	100%

X‑ray Scattering Factors (506)

Energy (eV)	f₁	f₂
10	—	0
10.1617	—	0
10.3261	—	0
10.4931	—	0
10.6628	—	0
10.8353	—	0
11.0106	—	0
11.1886	—	0
11.3696	—	0
11.5535	—	0

Additional Data

Estimated Crustal Abundance

The estimated element abundance in the earth's crust.

3.5 milligrams per kilogram

References (1)

[5] Argon https://education.jlab.org/itselemental/ele018.html

Estimated Oceanic Abundance

The estimated element abundance in the earth's oceans.

4.5×10-1 milligrams per liter

References (1)

[5] Argon https://education.jlab.org/itselemental/ele018.html

Sources

Sources of this element.

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.

References (1)

[6] Argon https://periodic.lanl.gov/18.shtml

References

(9)

1 PubChem

Data deposited in or computed by PubChem

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2 Atomic Mass Data Center (AMDC), International Atomic Energy Agency (IAEA)

The half-life and atomic mass data was provided by the Atomic Mass Data Center at the International Atomic Energy Agency.

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3 IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW)

Argon

Element data are cited from the Atomic weights of the elements (an IUPAC Technical Report). The IUPAC periodic table of elements can be found at https://iupac.org/what-we-do/periodic-table-of-elements/. Additional information can be found within IUPAC publication doi:10.1515/pac-2015-0703 Copyright © 2020 International Union of Pure and Applied Chemistry.

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4 IUPAC Periodic Table of the Elements and Isotopes (IPTEI)

The information are cited from Pure Appl. Chem. 2018; 90(12): 1833-2092, https://doi.org/10.1515/pac-2015-0703.

Visit source License

License note: Copyright (c) 2020 International Union of Pure and Applied Chemistry. The International Union of Pure and Applied Chemistry (IUPAC) contribution within Pubchem is provided under a CC-BY-NC-ND 4.0 license, unless otherwise stated.

5 Jefferson Lab, U.S. Department of Energy

Argon

Thomas Jefferson National Accelerator Facility (Jefferson Lab) is one of 17 national laboratories funded by the U.S. Department of Energy. The lab's primary mission is to conduct basic research of the atom's nucleus using the lab's unique particle accelerator, known as the Continuous Electron Beam Accelerator Facility (CEBAF). For more information visit https://www.jlab.org/

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License note: Please see citation and linking information: https://education.jlab.org/faq/index.html

6 Los Alamos National Laboratory, U.S. Department of Energy

Argon

The periodic table at the LANL (Los Alamos National Laboratory) contains basic element information together with the history, source, properties, use, handling and more. The provenance data may be found from the link under the source name.

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7 NIST Physical Measurement Laboratory

Argon

The periodic table contains NIST's critically-evaluated data on atomic properties of the elements. The provenance data that include data for atomic spectroscopy, X-ray and gamma ray, radiation dosimetry, nuclear physics, and condensed matter physics may be found from the link under the source name. Ref: https://www.nist.gov/pml/atomic-spectra-database

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8 PubChem Elements

Argon

This section provides all form of data related to element Argon.

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9 PubChem Elements

Argon

The element property data was retrieved from publications.

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Last updated: May 1, 2026

Data verified: May 12, 2026

Content is reviewed against latest scientific data.