Correct Option
The correct option isTheir outermost shells are completely filled.
Explanation
Chemical reactivity is primarily determined by the electronic configuration of an atom's outermost (valence) shell. Atoms tend to react with other atoms to achieve a stable electronic configuration, typically characterized by a fully filled valence shell (following the octet or duplet rule).
Detailed Analysis
- They have a high number of neutrons. is Incorrect: The number of neutrons affects the atomic mass and nuclear stability (isotopes) but has no direct role in determining chemical reactivity or bonding behavior.
- Their outermost shells are completely filled. is Correct: Inert gases (Noble gases) such as Helium and Neon possess completely filled outermost shells. Helium has a stable duplet ($1s^2$), and Neon has a stable octet ($2s^2 2p^6$). Because their valence shells are full, they have high ionization enthalpies and positive electron gain enthalpies, resulting in no tendency to lose, gain, or share electrons under normal conditions.
- They do not contain protons. is Incorrect: All atoms consist of protons, neutrons, and electrons. The presence of protons defines the atomic number and identity of the element; it does not imply a lack of protons.
- They are isotopes of each other. is Incorrect: Isotopes are atoms of the same element with different numbers of neutrons. Helium and Neon are distinct elements with different atomic numbers ($Z=2$ and $Z=10$, respectively), not isotopes of each other.
Key Takeaway: The chemical inertness of Group 18 elements (Noble gases) is due to their stable, closed-shell electronic configurations ($ns^2 np^6$, or $1s^2$ for Helium), which prevents them from participating in chemical bonding under standard conditions.