Argon

chlorine ← argon → potassium Ne ↑ Ar ↓ Kr 18Ar Periodic table
Appearance
colorless gas exhibiting a lilac/violet glow when placed in a high voltage electric field Vial containing a violet glowing gas Spectral lines of argon
General properties
Name, symbol, number	argon, Ar, 18
Pronunciation	/ˈɑrɡɒn/
Element category	noble gases
Group, period, block	18, 3, p
Standard atomic weight	39.948(1) g/mol
Electron configuration	[Ne] 3s2 3p6
Electrons per shell	2, 8, 8 (Image)
Physical properties
Phase	gas
Density	(0 °C, 101.325 kPa) 1.784 g/L
Liquid density at b.p.	1.40 g/cm3
Melting point	83.80 K, −189.35 °C, −308.83 °F
Boiling point	87.30 K, −185.85 °C, −302.53 °F
Triple point	83.8058 K (-189°C), 69 kPa
Critical point	150.87 K, 4.898 MPa
Heat of fusion	1.18 kJ/mol
Heat of vaporization	6.43 kJ/mol
Specific heat capacity	(25 °C) 5R/2 = 20.786 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k at T (K)   47 53 61 71 87
Atomic properties
Oxidation states	0
Electronegativity	no data (Pauling scale)
Ionization energies	1st: 1520.6 kJ/mol
	2nd: 2665.8 kJ/mol
	3rd: 3931 kJ/mol
Covalent radius	106±10 pm
Van der Waals radius	188 pm
Miscellanea
Crystal structure	face-centered cubic
Magnetic ordering	diamagnetic
Thermal conductivity	(300 K) 17.72x10-3  W/(m·K)
Speed of sound	(gas, 27 °C) 323 m/s
CAS registry number	7440–37–1
Most stable isotopes
Main article: Isotopes of argon
iso NA half-life DM DE (MeV) DP 36Ar 0.337% 36Ar is stable with 18 neutrons 37Ar syn 35 d ε 0.813 37Cl 38Ar 0.063% 38Ar is stable with 20 neutrons 39Ar trace 269 y β− 0.565 39K 40Ar 99.600% 40Ar is stable with 22 neutrons 41Ar syn 109.34 min β− 2.49 41K 42Ar syn 32.9 y β− 0.600 42K

Argon is a chemical element. The symbol for argon is Ar, and its atomic number (or proton number) is 18. It is a noble gas and no electrons or protons can be lost or gained from this atom.

Argon atoms are found in air. About 1% of the Earth's atmosphere (the air around us) is argon.

Related pages

• List of common elements
• Periodic table