18
Ar
Argon
39.948
Argon (Ar)
Noble GasGroup 18Period 3p-block-block
Argon is a chemical element with symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most common gas in the Earth's atmosphere, at 0.934% (9,340 ppmv), making it over twice as abundant as the next most common atmospheric gas, water vapor (which averages about 4000 ppmv, but varies greatly), and 23 times as abundant as the next most common non-condensing atmospheric gas, carbon dioxide (400 ppmv), and more than 500 times as abundant as the next most common noble gas, neon (18 ppmv).
Latin name: Argon
Atomic Number
18
Atomic Mass
39.948 g/mol
Phase (STP)
Gas
Electron Config.
[Ne] 3s² 3p⁶
General
SymbolAr
Atomic Number18
Atomic Mass39.948 g/mol
CategoryNoble Gas
Group18
Period3
Blockp-block
Electron Configuration[Ne] 3s² 3p⁶
Valence Electrons8
ColourColorless gas
Appearancecolorless gas exhibiting a lilac/violet glow when placed in a high voltage electric field
CAS NumberCAS7440-37-1
Physical Properties
Phase (STP)Gas
Density0.001784 g/cm³
Melting Point-189.35°C · -308.83°F · 83.80 K
Boiling Point-185.85°C · -302.53°F · 87.30 K
Atomic Properties
Atomic Radius71 pm
Covalent Radius106 pm
Van der Waals Radius188 pm
Ionization Energy1521 kJ/mol
Electromagnetic
Electrical TypeInsulator
Abundance
In Universe0.0002%
In Sun0.000002%
In Earth's Crust0.0003%
In Oceans0.000000045%
In Humans0%
In Meteorites0%
Discovery & History
Discovery Year1894
Discovered ByLord Rayleigh
Isotopes of Argon
| Isotope | Abundance | Half-life | Decay |
|---|---|---|---|
| Ar-36 | 0.3365% | Stable | — |
| Ar-38 | 0.0632% | Stable | — |
| Ar-39 | — | 269 years | β⁻ |
| Ar-40 | 99.6003% | Stable | — |
| Ar-42 | — | 32.9 years | β⁻ |