yawiki.org entry for Noble gas

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The noble gases are the chemical elements in group 18 (old-style Group 0) of the periodic table. This chemical series contains helium, neon, argon, krypton, xenon, radon, and ununoctium.

    Noble gas
        Etymology
        Chemistry
        Applications
        Physical Properties
        Notes

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Etymology
The noble gases were previously referred to as inert gases, but this term is not strictly accurate because several of them do take part in chemical reactions. Another older term was rare gases, although in fact argon forms a considerable part (0.93% by volume, 1.29% by mass) of the Earth's atmosphere.

The name 'noble gases' is an allusion to the similarly unreactive Noble metals, so called due to their preciousness, resistance to corrosion and long association with the aristocracy.

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Chemistry
The existence of noble gases was not known until after the advent of the periodic table. In the late nineteenth century, Lord Rayleigh discovered that some samples of nitrogen from the air were of a different density than nitrogen resulting from chemical reactions. He and William Ramsay theorized that the nitrogen extracted from air was associated with another gas, argon. With this discovery, they realized that a whole class of gases was missing from the periodic table. Eventually all the known noble gases were discovered and the table was completed. Noble gases have full valence electron shells. 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.

Under standard conditions, the noble gases all occur as monatomic gases.

All of them exihibit an extremely low chemical reactivity, in fact no conventional compounds of helium or neon have yet been prepared. Xenon and krypton are known to show some reactivity in the laboratory—see noble gas compounds . 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.

Because of their unreactivity, the noble gases were not discovered until 1868, when helium was detected spectrographically in the Sun. The isolation of helium on Earth had to wait until 1895. 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 gases under normal conditions, even those with larger atomic masses than many normally solid elements.

Ununoctium

Ununoctium is expected to be another noble gas, filling the empty space in the periodic table beneath radon. All its isotopes are likely to be radioactive with a very short half-life in the millisecond range.

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Applications
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 unreactivity (compared to flammable hydrogen) and lightness, is often used in blimps and balloons.

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Notes

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