Argon (Ar)
Argon (Ar) is a noble gas with atomic number 18 and a standard atomic weight of 39.95. At room temperature it is a gas that melts at -189 °C and boils at -186 °C.
Argon key facts
Argon at a glance
Identity
| Symbol | Ar |
|---|---|
| Atomic number (Z) | 18 |
| Atomic weight | 39.95 ± 0.16 (CIAAW 2024) |
| Category | Noble gas |
| Group | 18 — noble gases |
| Period | 3 |
| Block | p-block |
| CAS number | 7440-37-1 |
Atomic properties
| Electron configuration | [Ne] 3s2 3p6 Full: 1s2 2s2 2p6 3s2 3p6 |
|---|---|
| Electrons per shell | 2, 8, 8 |
| First ionisation energy | 15.76 eV (1,520.6 kJ/mol) 2nd: 27.63 eV · 3rd: 40.735 eV |
| Atomic radius (empirical) | 71 pm |
| Covalent radius | 96 pm |
| Van der Waals radius | 188 pm |
Physical properties
| State at 25 °C | Gas |
|---|---|
| Melting point | -189.3 °C · -308.8 °F · 83.81 K |
| Boiling point | -185.8 °C · -302.5 °F · 87.3 K |
| Density | 1.784 g/L (gas at 0 °C, 1 atm) |
| Specific heat capacity | 0.52 J/(g·K) |
| Thermal conductivity | 0.018 W/(m·K) |
| Crystal structure | Face-centred cubic |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | No |
| Natural isotopes (abundance) | 40Ar 99.6035%, 36Ar 0.3336%, 38Ar 0.0629% |
| Stable isotopes | 4 |
| Known isotopes | 26 |
| Abundance in Earth's crust | 3.5 mg/kg |
| Abundance in seawater | 0.45 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Argon electron configuration
Argon has 18 electrons. Written in short form, its ground-state configuration is [Ne] 3s2 3p6 — the electrons of the noble gas neon, plus 3s2 3p6.Written out in full: 1s2 2s2 2p6 3s2 3p6.
As a member of group 18 it has 8 valence electrons in its outer shell (shell 3), which largely decide its chemistry — a full outer shell, which is why it is so unreactive.
Discovery and name
1894 — Lord Rayleigh and William Ramsay · England.
Greek argos, idle or lazy — it hardly reacts.
Known for
About 0.93% of air — the third most abundant atmospheric gas. Used as an inert shield gas in welding, in light bulbs and in double-glazed windows.
Isotopes of argon
Naturally occurring argon is a mix of 3 isotopes. Each has 18 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 40Ar | 18 | 22 | 99.6035% | stable |
| 36Ar | 18 | 18 | 0.3336% | stable |
| 38Ar | 18 | 20 | 0.0629% | stable |
26 isotopes of argon are known in our data set (half-lives as published in nuclear data tables).
Argon compared with group 18
The other elements in group 18 (noble gases), from top to bottom of the table:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Helium | 2 | 4.0026 | — | -269 °C | 0.1786 g/L | — |
| Neon | 10 | 20.180 | -249 °C | -246 °C | 0.9002 g/L | — |
| Argon | 18 | 39.95 | -189 °C | -186 °C | 1.784 g/L | — |
| Krypton | 36 | 83.798 | -157 °C | -153 °C | 3.749 g/L | 3 |
| Xenon | 54 | 131.29 | -112 °C | -108 °C | 5.894 g/L | 2.6 |
| Radon | 86 | [222] | -71 °C | -62 °C | 9.73 g/L | 2.2 |
| Oganesson | 118 | [294] | — | — | 7 g/cm³ (pred.) | — |