Rare gases are stable because they

Analysis

The question asks for the fundamental reason behind the chemical stability (or inertness) of rare gases (also known as noble gases or Group 18 elements).

Concept Explanation

In chemistry, the stability of an atom is largely determined by its electronic configuration. Atoms generally react with one another to achieve a stable state, which is defined by having a "full" outermost shell.

• The Octet Rule: Most elements strive to have 8 electrons in their outermost shell (an octet) to achieve stability, similar to the noble gases.
• The Duplet Rule: For the smallest element, Helium (He), stability is achieved with 2 electrons in its outermost shell (a duplet).
• Noble Gases: Elements like Helium, Neon, Argon, Krypton, Xenon, and Radon already possess these full outer shells naturally. Because their outermost shells are completely filled, they have no tendency to gain, lose, or share electrons. This makes them chemically inert or "stable."

Step-by-Step Solution

1. Evaluate Option A: While it is true that rare gases are monoatomic, being monoatomic is a result of their stability, not the cause of it.
2. Evaluate Option B: Rare gases do not form ions easily. In fact, they are known for their extremely high ionization energies, meaning they resist losing electrons.
3. Evaluate Option C: This describes the electronic structure of the noble gases. Because they have a full duplet (for Helium) or a full octet (for the others), they have no "vacancies" in their valence shells. This lack of vacancy prevents them from reacting with other elements under normal conditions.
4. Evaluate Option D: Being volatile is a physical property related to boiling points and intermolecular forces, but it does not explain why they are chemically stable.

Conclusion

The stability of rare gases is a direct consequence of their electronic configuration. Having a full outer shell (duplet or octet) means they have reached the lowest possible energy state, making them unreactive.

Exam Tip: Whenever you see a question about the reactivity of elements, always look for the option that mentions the "outermost shell" or "valence electrons." Elements react specifically to complete their octet; if they already have it, they stay stable!