Noble gas aided in the development of a general understanding of atomic structure. In 1895, French chemist Henri Moissan attempted to form a reaction between fluorine, the most electronegative element, and argon, one of the noble gases, but failed. Scientists were unable to prepare compounds of argon until the end of the 20th century, but these attempts helped to develop new theories of atomic structure. Learning from these experiments, Danish physicist Niels Bohr proposed in 1913 that the electrons in atoms are arranged in shells surrounding the nucleus, and that for all noble gases except helium the outermost shell always contains eight
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.