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
Atmosphere of Uranus from the analysis of "Voyager 2" far infrared and radio occultation observations. The currently accepted value is in the upper troposphere, which corresponds to a mass fraction . This value is very close to the protosolar helium mass fraction of , indicating that helium has not settled towards the centre of the planet as it has in the gas giants. The third most abundant constituent of the Uranian atmosphere is methane , the presence of which has been known for some time as a result of the ground-based spectroscopic observations. Methane possesses prominent absorption bands in the visible and near-infrared,