Argon

Argon (Ar) is a noble gas with atomic number 18 and an atomic mass of 39.95. Its electron configuration is [Ne] 3s2 3p6, which gives it 8 valence electrons.

Argon key data

Symbol	Ar
Atomic number	18
Atomic mass (standard atomic weight)	39.95
Molar mass	39.95 g/mol
Protons	18
Neutrons (most abundant isotope, argon-40)	22
Electrons (neutral atom)	18
Valence electrons	8
Category	Noble gas
Period	3
Group	18
Block	p-block
Noble gas notation	[Ne] 3s2 3p6
Full electron configuration	1s2 2s2 2p6 3s2 3p6
Electrons per shell	2, 8, 8

Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.

Measured properties of argon

From PubChem, the US National Institutes of Health. Public domain. PubChem’s periodic table gives no value for the rows not shown.

Melting point	83.8 K (−189.35 °C), 97th of 103 by melting point
Boiling point	87.3 K (−185.85 °C), 88th of 93 by boiling point
Density	0.001784 g/cm³, 90th of 96 by density
State at room temperature	Gas
Van der Waals radius	188 pm, 73rd of 99 by atomic radius
First ionisation energy	15.76 eV (1521 kJ/mol), 4th of 102 by first ionisation energy
Common oxidation states	0
Discovered	1894

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Argon electron configuration

The full electron configuration of argon is 1s2 2s2 2p6 3s2 3p6. With the noble-gas core collapsed it is [Ne] 3s2 3p6, and the outermost subshell is 3p6.

Bohr model of argon (Ar): a nucleus and three shells holding 2, 8 and 8 electrons from the inside out. — Bohr model of argon (Ar). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Orbital box diagram of argon (Ar), 1s2 2s2 2p6 3s2 3p6: one box per orbital, filled by Hund’s rule, showing every orbital drawn holds a pair of electrons. — Orbital diagram of argon (Ar). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one argon atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

one atom = 39.95 / (6.02214076 × 10²³)
= 6.6339 × 10⁻²³ g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 39.95
= 1.5074 × 10²² atoms

Argon on the periodic table

Argon is in period 3, group 18, in the p-block.

The other elements of group 18 are helium, neon, krypton, xenon, radon and oganesson.