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The noble gases are a chemical series. They are the elements in group 18 (old-style Group 0) of the periodic table; specifically helium, neon, argon, krypton, xenon and radon.
The noble gases were previously referred to as inert gases, but this term is not strictly accurate now that some have been shown to take part in chemical reactions. Another older term, was the name rare gases, although in fact they form a considerable part (1 to 2 % by volume) of the Earth's atmosphere.
Because of their unreactivity, the noble gases were not discovered until the existence of helium was hypothetically deduced from a spectrographic analysis of the sun, and later on proven when William Ramsay isolated it. The noble gases also 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 of them which have much heavier atoms than many normally solid elements.
These elements all have full s and p outer electron shells (i.e. 8 outer shell electrons), and so do not form chemical compounds easily. As the atoms get larger down the series, they become (slightly) more reactive, and xenon has been induced to form a number of compounds with fluorine. In 1962, Neil Bartlett, while working at the University of British Columbia, reacted xenon with fluorine to produce XeF2, XeF4, and XeF6 compounds. Radon has reacted with fluorine to form radon fluoride (RnF) which glows with a yellow light in the solid state. Additionally, krypton is able to react with fluorine to form KrF2, and short-lived excimers of Xe2 and noble gas halides such as XeCl can be made, which are used in excimer lasers.
In 2002, compounds were discovered where uranium forms molecules with argon, krypton, or xenon. This suggests that the noble gases may be able to form compounds with other metals too.
The fluoride of argon (ArF2) was discovered in 2003 from the swiss chemist Helmut Durrenmatt.
Also notice that there is a an empty space on the periodic table under Radon. This means that there could theoretically be another undiscovered noble gas, which has been temporarily named Ununoctium.
External link - Ohio State University press release (http://www.osu.edu/researchnews/archive/noblegas.htm) for uranium compounds with noble gases.
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