Ar
Argon
Element 18 · Noble gas
Argon (Ar)
Argon is an inert noble gas and the third most common gas in Earth’s atmosphere, making up almost 1% of the air.
- Noble gas
- Period 3
- Group 18
- p-block
- gas at room temperature
Key properties
- Atomic mass
- 39.9
- u
- Electron configuration
- [Ne] 3s2 3p6
- Melting point
- -189.3 °C
- 83.8 K
- Boiling point
- -185.8 °C
- 87.3 K
- Density
- 1.7837 g/L
- Electronegativity
- —
- Atomic radius
- 188 pm
- van der Waals
- 1st ionisation energy
- 15.76 eV
Compare with every element:Electronegativity chartAtomic radius chartIonisation energy chartDensity chart of the elementsMelting point chartElectron configuration chart
Did you know?
Notable facts about argon
- 01
Rayleigh noticed nitrogen from air was slightly denser than nitrogen made chemically — the difference was argon.
- 02
Most of Earth’s argon is argon-40, made by the radioactive decay of potassium-40 in rocks.
- 03
The discovery earned Nobel Prizes for both Rayleigh and Ramsay in 1904.
Applications
What is argon used for?
- Shielding gas for welding
- Filling incandescent and fluorescent bulbs
- Insulating double-glazed windows
- Preserving wine and documents
History
Discovery & name origin
Discovered by
Year
Where
England
Name origin
Greek argos, “lazy” or “idle” — it refuses to react.
Atomic structure
Atomic properties
Atomic structure
Atomic properties
- Atomic number
- 18
- Atomic mass
- 39.9 u
- Group
- 18
- Period
- 3
- Block
- p-block
- Electron configuration
- [Ne] 3s2 3p6
- Full configuration
- 1s2 2s2 2p6 3s2 3p6
- Electrons per shell
- 2, 8, 8
- Electronegativity (Pauling)
- —
- Electron affinity
- —
- Atomic radius (van der Waals)
- 188 pm
- Stable isotopes
- 3
Oxidation states
Highlighted: common oxidation states (0).
Successive ionisation energies
Energy to remove each electron in turn (kJ/mol). Big jumps show where a new, inner shell begins.
- 1st1,520.6
- 2nd2,665.8
- 3rd3,931
- 4th5,771
- 5th7,238
- 6th8,781
- 7th11,995
- 8th13,842
Physical
Physical properties & colour
Physical
Physical properties & colour
- State at 20 °C
- Gas
- Melting point
- 83.8 K · -189.3 °C · -308.8 °F
- Boiling point
- 87.3 K · -185.8 °C · -302.5 °F
- Density
- 1.7837 g/L
- Molar heat capacity
- —
- Appearance
- Colourless gas exhibiting a lilac/violet glow when placed in a high voltage electric field
- CPK model colour
- Copied
The CPK colour is the convention used for this element’s atoms in molecular models.
FAQ
Argon questions
What is Ar on the periodic table?
Ar is the symbol for argon, element 18 on the periodic table. It sits in group 18, period 3, in the noble gas category, and has an atomic mass of 39.9.
What is the atomic number of argon?
Argon has atomic number 18, so every argon atom has 18 protons in its nucleus. Its symbol is Ar and its atomic mass is 39.9.
Who discovered argon?
Argon was discovered by Lord Rayleigh and William Ramsay in 1894 (England). The name comes from Greek argos, “lazy” or “idle” — it refuses to react.
What is argon used for?
Argon is used for: shielding gas for welding; filling incandescent and fluorescent bulbs; insulating double-glazed windows; preserving wine and documents.
Is argon a metal?
No. Argon is classed as a noble gas.
What is the electron configuration of argon?
The electron configuration of argon is [Ne] 3s2 3p6. Its 18 electrons are arranged in shells as 2, 8, 8.