noble gases (He, Ne, Ar, Kr, Xe, Rn) are the elements with the fully filled outer shell of valence electrons, so that under normal conditions their atoms do not need to add or deliver any electrons to reach the most stable, so-called octet configuration, as they already have it. So they cannot be involved in most chemical reactions under normal conditions. For ages, in chemistry the knowledge about the elements was based on their reactivity and properties like color, taste, appearance etc.; and as noble gases are colorless, odorless, tasteless and nonflammable under normal conditions, and do not react with other elements except under the extreme conditions, nothing was known about them until some circumstantial evidences were found, and only at the end of the 19 th century noble gases were successfully isolated from air.
For a long time it was believed that these elements have zero valence and simply cannot form any compounds with other elements. The other reason was that with the technical equipment available at the time, scientists were not able to create the conditions under which noble gases could form compounds. Only in 1962 English chemist Neil Bartlett discovered the first compound of noble gas, namely xenon hexafluoroplatinate. Later other compounds have been discovered, proving that noble gases indeed can form compounds. The reactivity of noble gases increases from helium to radon, because in heavier noble gases the electrons of the outer shell are located on bigger distance from the nucleus and can be snapped off easier. Up to now no covalent compounds of helium and neon are known. For a long time only compounds of noble gases with oxygen and fluorine were known; these two elements are the most reactive and electronegative in the periodic table, which means that they have the best chances to snap off the electrons from the atoms of noble gases and form covalent bonds thereof; this is the explanation to the given question. But we have to say that up to now the compounds of noble gases which contain bonds between noble gas atoms and atoms of elements different from oxygen and fluorine are already known, although most of them are very unstable at normal conditions and can be extracted only at very low temperatures and some other special conditions. We have to say that now chemistry of noble gases and their compounds becomes one of the very interesting and promising fields of modern chemistry.