Noble gases have a complete outer shell of electrons because they have attained the electronic configuration known as the "octet rule" or "2-8 rule." This configuration is characterized by having eight valence electrons, except for helium, which has two valence electrons.
The outer shell of an atom is called the valence shell, and it determines the chemical behavior of the atom. Noble gases, such as helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn), have completely filled valence shells. For example, helium has two electrons in its valence shell (1s^2), while the other noble gases have eight electrons in their valence shells (2s^2 2p^6). This electron configuration provides a high degree of stability.
The stability of noble gases is attributed to the fact that their completely filled valence shells fulfill the octet rule. According to this rule, atoms tend to gain, lose, or share electrons to achieve a stable electron configuration similar to that of the noble gases. By doing so, they achieve a full outer shell, which corresponds to a more stable and energetically favorable state.
Noble gases are highly unreactive due to their stable electron configurations. They have little tendency to gain or lose electrons or to form chemical bonds with other elements. This inertness is a result of the high energy required to either remove or add electrons to their already stable configurations.
Overall, noble gases have complete outer shells due to their electron configurations following the octet rule, providing them with exceptional stability and low reactivity.