Noble gases are unreactive because they have the full complement of electrons in their outermost energy levels. This full valence shell gives these atoms the most stable configuration they can achieve. Elements react with one another to gain a full complement of electrons like the noble gases.
All of the noble gases, except for helium, contain eight electrons in their outermost energy levels. Helium has two electrons in its outermost energy level, the maximum number that its single energy level can hold. Because these elements already contain the maximum number of electrons in their outermost energy levels, they do not need to find other atoms with which to form compounds. The noble gases were given this name because they do not interact much with other elements. Because of their inherent stability, noble gases can be found in their free forms in nature.