Rare gases, also known as noble gases, are stable because they have a complete outermost shell of electrons. In other words, they have octet structures, meaning they have eight electrons in their outermost energy level, with the exception of helium, which has two electrons in its outermost energy level. This full outer shell makes them highly unreactive, since they have no need to gain or lose electrons to achieve a more stable configuration. Therefore, rare gases do not readily form chemical bonds with other elements, making them very stable.