Elements Group 18, Helium family, noble gases
For the first six periods of the periodic table, the
noble gases are exactly the members of group 18. Noble gases
are typically highly unreactive except when under particular
extreme conditions. The inertness of noble gases makes them
very suitable in applications where reactions are not
wanted.
The properties of the noble gases can be well explained
by modern theories of atomic structure: their outer shell of
valence electrons is considered to be "full", giving them
little tendency to participate in chemical reactions, and it
has been possible to prepare only a few hundred noble gas
compounds. The melting and boiling points for a given noble
gas are close together, differing by less than 10 °C; that is,
they are liquids over only a small temperature range.
Neon, argon, krypton, and xenon are obtained from air in
an air separation unit using the methods of liquefaction of
gases and fractional distillation. Helium is sourced from
natural gas fields which have high concentrations of helium
in the natural gas, using cryogenic gas separation
techniques, and radon is usually isolated from the
radioactive decay of dissolved radium, thorium, or uranium
compounds.
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