 The noble gases, or 'inert' gases, are the naturally occurring elements of group 18, in light blue on this periodic table. They are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). Under standard conditions, these elements are odourless, colourless, tasteless, and nonflammable monatomic gases with very low chemical reactivity. None of the elements in this group has any biological importance. The noble gases' tendency not to react with other chemical substances results from their electron configuration: their outer shell of valence electrons is 'full', giving them little tendency to participate in chemical reactions. Only a few hundred noble gas compounds are known to exist, many involving xenon. The inertness of noble gases makes them useful whenever chemical reactions are unwanted. For example, argon is used as a shielding gas in welding and as a filler gas in incandescent light bulbs. Helium is used to provide buoyancy in blimps and balloons. Helium and neon are also used as refrigerants due to their low boiling points. Industrial quantities of the noble gases, except for radon, are obtained by separating them from air. Helium is also a byproduct of the mining of natural gas. Radon is usually isolated from the radioactive decay of radium, thorium, or uranium compounds. 
Noble gases can form carbon fullerene compounds, in which the noble gas atom is trapped inside a fullerene molecule. At the right is a representation of the structure of a noble-gas atom caged within a buckminsterfullerene (C60) molecule. The abundances of the noble gases in the universe decrease as their atomic numbers increase. Helium is the most common element in the universe after hydrogen. Most of the helium in the universe was formed during Big Bang, but the amount of helium is steadily increasing due to the fusion of hydrogen in stellar nucleosynthesis. It is also created by the alpha decay of heavy elements such as uranium and thorium found in the Earth's crust, and tends to accumulate in natural gas deposits. Helium is only the third most abundant noble gas in Earth's atmosphere. The reason is that because of the small mass of the atom, helium cannot be retained by the Earth's gravitational field. Radon is the least plentiful noble gas, but is constantly being formed in the lithosphere by the alpha decay of radium. Radon gas can seep into buildings through cracks in their foundations and accumulate in areas that are not well ventilated. Due to its high radioactivity, radon presents a significant health hazard; it is implicated in an estimated 21,000 lung cancer deaths per year in the United States alone. |