| Elements belonging to Group 18 of the periodic table. These elements are very unreactive, however, they are not completely nonreactive as compounds containing these elements have been synthesized. Chemically, they are very stable due to having the maximum number of valence
electrons their outer shell can hold. Valence electrons are the outermost electrons of an atom and are normally the only electrons
which can participate in chemical bonding. According to atomic theory derived from quantum mechanics and experimental trends,
atoms with full valence electron shells are extraordinarily stable and therefore do not form chemical bonds. There are no naturally occurring compounds that
are made up of these elements.
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| All of them exhibit an extremely low chemical reactivity and very few noble gas compounds have been prepared. No conventional
compounds of helium or neon have yet been prepared, while xenon and krypton are known to show some reactivity in the laboratory. Recently argon compounds have also been successfully characterised.
The noble gases' lack of reactivity can be explained in terms of them having a "complete valence shell". They have little
tendency to gain or lose electrons. The noble gases have high ionization energies and negligible electronegativities. The
noble gases have very weak inter-atomic forces of attraction, and consequently very low melting points and boiling points. This is why they are all monatomic gases under normal conditions, even those with larger atomic masses than many normally solid elements.
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| One of the most commonly encountered uses of the noble gases in everyday life is in lighting. Argon is often used as a suitable safe and inert atmosphere for the inside of filament light bulbs, and is also used as an inert
atmosphere in the synthesis of air and moisture sensitive compounds (as an alternative for nitrogen). Some of the noble gases glow distinctive colours when used inside lighting tubes (neon lights). Helium, due to its nonreactivity (compared with flammable hydrogen) and lightness, is often used in blimps and balloons. Helium and Argon are very commonly used to shield a welding arc, and the surrounding base metal from the atmosphere during welding. Krypton is also used in lasers, which are used by doctors for eye surgery.
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