Noble gases, also called inert or rare gases are composed of atoms that have filled electron shells and no valence electrons. The exception is Helium which has one pair of electrons. Filled electron shells are extremely stable and have high ionization potentials. Noble gases generally have very low boiling points and their physical properties vary in a systematic way with atomic number. Boiling points increase with increasing atomic number, which is a consequence of the increasing vaporization heats.
The vaporization heat is the amount of work, necessary to overcome the interatomic forces of attraction. As between atoms of noble gases common interactions do not exist, compared with the forces involved in a covalent bond these weak forces of interaction must be forces of Van der Waals or London, and are inversely proportional to the ionization potential of the element. Therefore, these forces increase with the size of the electronic cloud and also they increase as electronic cloud becomes more diffuse.
The capacity of noble gases to enter in combination with other atoms is very limited. Up to now it has been possible to prepare compounds of krypton, xenon and radon. In experiments with tracers the existence of a radon fluoride has settled down, but its composition is ignored. The energy released in the radioactive decay of the radon enormously makes difficult to the establishment of its own stability and reactivity.
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