Argon (Ar)
Element 18 · Noble gas · atomic mass 39.95 u · group 18, period 3
Electron shells: 2, 8, 8
Argon is the third most common gas in the atmosphere and the cheapest inert gas to obtain. Its uselessness for reacting is precisely what makes it useful.
Nearly all the argon in the air came from potassium-40 decaying inside rocks.
- Atomic number
- 18
- Atomic mass
- 39.95 u
- Electron configuration
- [Ne] 3s² 3p⁶
- Group · period · block
- 18 · 3 · p
- Oxidation states
- 0
- Electronegativity
- Not measured
- Melting point
- -189 °C
- Boiling point
- -186 °C
- Density
- 0.0017837 g/cm³
- State at 20 °C
- Gas
- Appearance
- Colourless gas
- Discovered
- Lord Rayleigh and William Ramsay, 1894
18 protons, and 18 electrons when neutral
IUPAC interval [39.792, 39.963]
Noble gases
What Argon is used for
- Shielding gas for welding
- Filling incandescent and fluorescent bulbs
- Double-glazing insulation
- Preserving documents and wine
Atomic structure of Argon
A neutral argon atom has 18 protons and 18 electrons. Its most common isotope carries about 22 neutrons, which is where the mass number 40 comes from. Change the neutron count and you have a different isotope of argon; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 8. In orbital notation that is [Ne] 3s² 3p⁶, leaving 8 valence electrons in the outer shell — the ones that do the chemistry.
3D electron orbitals
See where Argon's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Argon sits in the periodic table
Group 18, with full outer shells and almost no drive to react. Their inertness is exactly what makes them useful for shielding, lighting and cooling.
Discovery and naming
Argon was discovered by Lord Rayleigh and William Ramsay in 1894. Its name comes from Greek argos, "idle".
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Argon?
The standard atomic weight of Argon is 39.95 u. Because the natural isotope mix varies between sources, IUPAC publishes it as the interval [39.792, 39.963], and 39.95 is the conventional single value to use in calculations.
How many protons, neutrons and electrons does Argon have?
Argon has 18 protons, because the atomic number is the proton count and that is what makes an atom Argon at all. A neutral Argon atom has the same number of electrons, 18. The neutron count varies by isotope; the most common one has about 22, since 40 − 18 = 22.
What is the electron configuration of Argon?
The electron configuration of Argon is [Ne] 3s² 3p⁶. Its electrons are arranged 2, 8, 8 across 3 shells.
Is Argon a metal?
No. Argon is classified as a noble gas, on the right-hand side of the table where elements gain or share electrons rather than losing them.
What group and period is Argon in?
Argon is in group 18 (the noble gases), period 3, in the p-block. The group is its column and the period is its row.
What is the melting point of Argon?
Argon melts at -189 °C and boils at -186 °C. That puts it in the gases at everyday temperatures.
What is Argon used for?
Shielding gas for welding, Filling incandescent and fluorescent bulbs, Double-glazing insulation, Preserving documents and wine. Argon is the third most common gas in the atmosphere and the cheapest inert gas to obtain.
Who discovered Argon, and where does its name come from?
Argon (Ar) was discovered by Lord Rayleigh and William Ramsay in 1894. The name comes from Greek argos, "idle".