What is the atomic number of argon?

Argon has atomic number 18, meaning its nucleus contains exactly 18 protons. It is the third noble gas in Group 18 of the periodic table, sitting between chlorine and potassium in Period 3.

What is the electron configuration of argon?

Argon's full electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁶. All valence shell slots are filled, giving argon a complete octet in the third shell. This closed-shell configuration is the reference for the [Ar] noble gas core used in electron configuration notation for elements beyond argon.

What are the stable isotopes of argon?

Argon has three naturally occurring isotopes: argon-40 (⁴⁰Ar, ~99.6035%), argon-36 (³⁶Ar, ~0.3336%), and argon-38 (³⁸Ar, ~0.0629%). Argon-40 dominates because it is the daughter product of the radioactive decay of potassium-40, which has accumulated in Earth's atmosphere over geological time.

Why is argon used as a shielding gas in welding?

Argon is completely chemically inert and does not react with molten metals at high temperatures. When used as a shielding gas in TIG and MIG welding, it blankets the weld pool and prevents oxidation, nitridation, and contamination from atmospheric oxygen and nitrogen, resulting in cleaner, stronger welds.

Can argon form chemical compounds?

Under standard conditions, argon does not form any stable chemical compounds. Its ionization energy (15.76 eV) and complete valence shell prevent normal chemical bonding. However, weakly bound van der Waals clusters such as ArHF and the metastable argon fluorohydride (HArF) have been observed at extremely low temperatures in laboratory settings.

What is the boiling point of argon?

Argon boils at −185.85 °C (87.30 K) at standard atmospheric pressure. Liquid argon is used as a cryogenic refrigerant and as the target medium in direct dark matter detection experiments and neutrino detectors.

Where does atmospheric argon come from?

About 99% of atmospheric argon (⁴⁰Ar) has accumulated over billions of years from the radioactive decay of ⁴⁰K (potassium-40) in Earth's crust and mantle. The gas continuously outgasses from volcanic activity. Primordial argon-36 and argon-38 were present at Earth's formation.

What is argon's atomic mass?

The standard atomic weight of argon is 39.948 u (IUPAC 2021). This is notably higher than the atomic weight of the next element, potassium (39.098 u), which historically caused an anomaly in early attempts to order the periodic table by atomic mass rather than atomic number.

How is argon produced commercially?

Argon is obtained as a byproduct of the fractional distillation of liquid air. Air is liquefied by compression and cooling, then distilled: nitrogen boils off first, followed by argon, and finally oxygen. About 700,000 tonnes of argon are produced globally each year.

Argon Element Properties

Name: Argon Element Properties
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Author: Nham Vu

Complete reference for Argon (Ar, element 18): atomic data, electron configuration, isotopes, physical constants, thermodynamic properties, and industrial uses.

18 Ar 39.948

Argon

Noble Gas — Period 3, Group 18

Atomic Identity

Atomic Number

Symbol

Standard Atomic Wt.

39.948 u

IUPAC 2021

Period

Group

VIIIA

Block

p-block

CAS Number

7440-37-1

Discovery

Lord Rayleigh & William Ramsay

1894

Named After

Greek: argos

"lazy / inactive"

Electron Configuration

Full notation 1s² 2s² 2p⁶ 3s² 3p⁶

Noble gas shorthand [Ne] 3s² 3p⁶

Electrons per shell 2, 8, 8

Valence electrons 8

Unpaired electrons 0

Spin multiplicity 1 (singlet)

Orbital Diagram

Diamagnetic

All orbitals fully paired

Isotopes of Argon

Isotope	Symbol	Protons	Neutrons	Mass (u)	Natural Abundance	Stability
Argon-36	³⁶Ar	18	18	35.967545	0.3336%	Stable
Argon-38	³⁸Ar	18	20	37.962732	0.0629%	Stable
Argon-40	⁴⁰Ar	18	22	39.962383	99.6035%	Stable
Argon-37	³⁷Ar	18	19	36.966776	Trace	Unstable Electron capture, t½ = 35.01 d
Argon-39	³⁹Ar	18	21	38.964313	Trace	Unstable β⁻ decay, t½ = 269 yr
Argon-41	⁴¹Ar	18	23	40.964501	Trace	Unstable β⁻ decay, t½ = 109.34 min
Argon-42	⁴²Ar	18	24	41.963050	Trace	Unstable β⁻ decay, t½ = 32.9 yr

Argon-40 is unusually abundant because it is the stable end product of ⁴⁰K (potassium-40) electron capture and beta decay, which has been occurring throughout Earth's geological history.

Physical Properties

State at STP Gas

Color Colorless

Odor Odorless

Density (gas, 0 °C, 1 atm) 1.784 g/L

Density (liquid at bp) 1.3954 g/cm³

Melting Point −189.35 °C (83.81 K) at 68.9 kPa

Boiling Point −185.85 °C (87.30 K)

Triple Point 83.8058 K, 68.89 kPa

Critical Temperature −122.46 °C (150.69 K)

Critical Pressure 4.863 MPa

Heat of Vaporization 6.53 kJ/mol

Heat of Fusion 1.18 kJ/mol

Specific Heat (gas) 20.786 J/(mol·K)

Thermal Conductivity 0.01772 W/(m·K) at 300 K

Speed of Sound (gas) 319 m/s (0 °C)

Viscosity 22.7 μPa·s (25 °C)

Chemical Properties

Electronegativity (Pauling) N/A (noble gas)

Electron Affinity −1.0 eV (essentially 0)

1st Ionization Energy 1520.6 kJ/mol (15.759 eV)

2nd Ionization Energy 2665.8 kJ/mol

3rd Ionization Energy 3931 kJ/mol

Covalent Radius 106 pm

Van der Waals Radius 188 pm

Oxidation States 0 (standard)

Reactivity Chemically inert

Magnetic Ordering Diamagnetic

Molar Volume (STP) 22.392 L/mol

Known Compounds HArF (metastable, very low T)

Flammability Non-flammable

Ground State Quantum Numbers

Principal (n) 3 (outermost shell)

Azimuthal (l) 1 (p orbital)

Magnetic (mₗ) −1, 0, +1 (all filled)

Spin (mₛ) All ±½ pairs

Term symbol ¹S₀

Degeneracy 1 (singlet ground state)

Common Industrial Uses

Welding shielding gas TIG/MIG welding, prevents oxidation

Semiconductor fabrication Sputtering, ion implantation, plasma

Lighting Incandescent bulb fill, plasma displays

Cryogenics Liquid Ar cryostat (87 K range)

Dark matter detectors Liquid Ar target (DEAP, DarkSide)

Fire suppression Inert gas flooding systems (Inergen)

Food packaging (E938) Preserves freshness, displaces oxygen

Laser medium Ar-ion laser (488 nm, 514 nm lines)

Key Facts About Argon

Most Abundant Noble Gas on Earth

Argon makes up about 0.934% of Earth's atmosphere by volume, making it the third most abundant atmospheric gas after nitrogen and oxygen, and by far the most plentiful noble gas on our planet.

Potassium-Argon (K-Ar) Dating

Geologists use the decay of radioactive ⁴⁰K to ⁴⁰Ar to date rocks and minerals ranging from ~100,000 to billions of years old. The known half-life of ⁴⁰K (1.25 billion years) makes this one of the most powerful geochronological tools.

Welding and Metallurgy

Argon is the dominant shielding gas in TIG (tungsten inert gas) and plasma welding. Its inertness at high temperatures protects molten metal from atmospheric oxygen and nitrogen, yielding stronger, purer welds.

Argon-Ion Laser

The argon-ion laser emits powerful lines at 488 nm (blue) and 514.5 nm (green) by ionizing argon gas with an electric discharge. These lasers are used in fluorescence microscopy, retinal surgery, and holography.

Liquid Argon Particle Detectors

Large volumes of liquid argon serve as the detection medium in time projection chambers (TPCs) used to search for dark matter (DEAP-3600, DarkSide-50) and study neutrinos (DUNE, ArgoNeuT). Argon scintillates brightly at 128 nm when struck by particles.

Periodic Table Anomaly

When Mendeleev arranged elements by atomic mass, argon (39.948 u) appeared to be heavier than potassium (39.098 u), disrupting the expected chemical-family order. This anomaly was resolved only after Moseley established that atomic number — not mass — is the fundamental organizing principle.

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Summary

Complete reference for Argon (Ar, element 18): atomic data, electron configuration, isotopes, physical constants, thermodynamic properties, and industrial uses.

How it works

Browse the atomic identity section for symbol, atomic number, and standard atomic weight.
Check the electron configuration panel for orbital notation and quantum numbers.
Review the isotopes table for argon-36, argon-38, and argon-40 data including natural abundances.
Consult the physical properties panel for boiling point, density, and state at STP.
Use the chemical properties section to understand noble gas behavior and ionization energies.
Explore the key facts cards to learn about industrial and scientific applications.
Click any value to copy it to your clipboard instantly.

Use cases

Look up argon constants for chemistry homework or exams.
Verify atomic data when writing lab or engineering reports.
Reference isotope abundances for geochronology (K-Ar dating) calculations.
Check thermodynamic constants for cryogenic or gas-handling engineering.
Confirm electron configuration before writing spectroscopy or plasma physics notes.
Understand why argon is used as a shielding gas in welding and metallurgy.
Compare argon properties with other noble gases for chemistry coursework.
Quick-reference ionization energies for plasma physics or semiconductor fabrication.

Frequently Asked Questions

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Last updated: 2026-05-28 · Reviewed by Nham Vu