Noble gas 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. For example, argon is used in incandescent lamps to prevent the hot tungsten filament from oxidizing; also, helium is used in breathing gas by deep-sea divers to prevent oxygen, nitrogen and carbon dioxide (hypercapnia) toxicity. The properties of the noble gases can be well explained by modern theories of atomic structure: their outer shell of valence electrons
Global Atmosphere Watch warning system for chemical or physical changes in the Earth's atmosphere which could be cause for environmental concern. Such changes might involve a change in ozone, and therefore ultraviolet, levels, levels of greenhouse gases, or precipitation chemistry, the culprit in the world's acid rain woes. The GAW consists of a coordinated system of various components, prominent among which are: More than 65 countries currently host and operate the GAW's global or regional measurement stations. There are also "contributing stations" that furnish additional data. Lately, satellite programmes have also become important to the GAW, providing atmospheric data that complement ground measurements.