Argon (Ar)

Argon is a noble gas with atomic number 18, in period 3 and group 18 of the periodic table. Its atomic mass is 39.95 u and its electron configuration is [Ne] 3s² 3p⁶. Gas at 25 °C.

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Key facts

Atomic number	18
Atomic mass	39.95 u
Category	Noble Gas
Period	3
Group	18
Block	p
Electron configuration	[Ne] 3s² 3p⁶
Electrons per shell	2, 8, 8
Electronegativity (Pauling)	---
Oxidation states	---
State at 25 °C	Gas
Melting point	83.8 K−189.35 °C
Boiling point	87.3 K−185.85 °C
Density	1.784 g/L

Atomic properties

First ionization energy	15.76 eV
Electron affinity	0 kJ/mol
Atomic radius	71 pm
Covalent radius	106 pm
Van der Waals radius	188 pm
Common ions	---
Protons, neutrons, electrons	18, 22, 18

Physical and thermal properties

Molar volume	22.56 cm³/mol
Heat of fusion	1.18 kJ/mol
Heat of vaporization	6.5 kJ/mol
Specific heat	520.33 J/(kg·K)
Thermal conductivity	0.01772 W/(m·K)
Thermal expansion	---
Mohs hardness	---
Young’s modulus	---
Crystal structure	Face-centered cubic
Magnetic type	Diamagnetic

Identification and history

CAS number	7440-37-1
Latin name	Argon
Discovered by	L. Rayleigh, W. Ramsay · 1894
Named by	---

Natural isotopes

Isotope	Mass (u)	Abundance
³⁶Ar	35.967545	0.3336 %
³⁸Ar	37.962732	0.0629 %
⁴⁰Ar	39.962383	99.6035 %

Data checked against PubChem (NIH), IUPAC, the NIST Atomic Spectra Database and the IAEA (NUBASE and AME), the same data you see in the app.

A dash (---) means there is no reliable published measurement. The app never makes one up.

See Argon in the app

Orbital diagram with Hund’s rule, emission spectrum, shell model, every isotope, a real photo, and comparison with any other element. Free, no ads, works offline.

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