Argon

Argon is a chemical
element in the periodic
table that has the symbol Ne
Ar and atomic number 18.  Ar
The third noble gas, in   Kr
period 8, argon makes up
about 1% of the Earth's
atmosphere.                                    General
                          Name, Symbol, Number      Argon, Ar, 18
                          Chemical series           Noble gases
                          Group, Period, Block      18 (VIIIA), 3 , p
                          Density, Hardness         1.784 kg/m3 (273 K), NA
                          Appearance                Colorless
                                         Atomic Properties
                          Atomic weight             39.948 amu
Notable Characteristics
                          Atomic radius (calc.)     no data (71) pm
Argon is 2.5 times as     Covalent radius           97 pm
soluble in water as
nitrogen which is         van der Waals radius      188 pm
approximately the same    Electron configuration    [Ne]3s2 3p6
solubility as oxygen.
This chemically inert     e- 's per energy level    2, 8, 8

element is colorless and  Oxidation states (Oxide)  0 (unknown)
odorless in both its
liquid and gaseous forms. Crystal structure         Cubic face centered
There are no known true                  Physical Properties
chemical compounds that   State of matter           gas (nonmagnetic)
contain argon.
                          Melting point             83.8 K (-308.7 ¡F)
Applications              Boiling point             87.3 K (-302.4 ¡F)
It is used in lighting    Molar volume              22.56 ×10-3 m3/mol
since it will not react   Heat of vaporization      6.447 kJ/mol
with the filament in a    Heat of fusion            1.188 kJ/mol
lightbulb even under high
temperatures and other    Vapor pressure            NA
cases where diatomic      Speed of sound            319 m/s at 293.15 K
nitrogen is an unsuitable
(semi-)inert gas. Other                     Miscellaneous
uses;                     Electronegativity         no data (Pauling scale)
                          Specific heat capacity    520 J/(kg*K)
   * Used as an inert gas
     shield in arc        Electrical conductivity   no data
     welding and cutting, Thermal conductivity      0.01772 W/(m*K)
   * as a non-reactive    1st ionization potential  1520.6 kJ/mol
     blanket in the
     manufacture of       2nd ionization potential  2665.8 kJ/mol
     titanium and other
     reactive elements,   3rd ionization potential  3931 kJ/mol
   * as a protective      4th ionization potential  5771 kJ/mol
     atmosphere for
     growing silicon and  5th ionization potential  7238 kJ/mol
     germanium crystals.
   * Argon-39 has been    6th ionization potential  8781 kJ/mol
     used for a number of 7th ionization potential  11995 kJ/mol
     applications,
     primarily ice        8th ionization potential  13842 kJ/mol
     coring. It has also                 Most Stable Isotopes
     been used for ground
     water dating          iso     NA    half-life      DM    DE MeV   DP
                          36Ar  0.336%  Ar is stable with 18 neutrons
Argon is also used in
technical SCUBA diving to 38Ar  0.063%  Ar is stable with 20 neutrons
inflate the drysuit, due
to its nonreactive, heat  39Ar  {syn.}  269 y         Beta-  0.565   39K
isolating effect.         40Ar  99.6%   Ar is stable with 22 neutrons

History                   42Ar  {syn}   32.9 y        Beta-  0.600   42K

Argon (Greek argos           SI units & STP are used except where noted.
meaning "lazy") was
suspected to be present in air by Henry Cavendish in 1785 but wasn't
discovered until 1894 by Lord Rayleigh and Sir William Ramsay.

Occurrence

This gas is isolated through liquid air fractionation since the atmosphere
contains only 0.94% argon. The Martian atmosphere in contrast contains 1.6%
of Ar-40 and 5 ppm Ar-36.

Isotopes

The main isotopes of argon found on earth are Ar-40, Ar-36, and Ar-38.
Naturally occurring K-40 with a half-life of 1.250 x 109 years, decays to
stable Ar-40 (11.2%) by electron capture and by positron emission, and also
decays to stable Ca-40 (88.8%) by negatron emission. These properties and
ratios are used to determine the age of rocks.

In earth's atmosphere, Ar-39 is made by cosmic ray activity, primarily with
Ar-40. In the subsurface environment, it is also produced through
neutron-capture by K-39 or alpha emission by calcium. Argon-37 is produced
from the decay of calcium-40, the result of subsurface nuclear explosions.
It has a half-life of 35 days.

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